Focal equivalent and how it relates to zoom?

[camera-user]

Hi,

I am wondering about changing my actual camera and I would like to understand the concept of focal equivalent.
Indeed, my current camera is the hx60 which has a 1/2.3 sensor and a 24-720mm zoom lens.
So, if my computation are rights, when I am zoomed at maximum, i.e. 720mm, this would translate to 2595mm for an APS-C, up to my knowledge there is no such lens on the market (and I guess I understand why).

Computation is one thing but on the field is another.
So, in practice, let's say I am at 30 meters of a paper which has a 1 cm letter printed on it.
If I zoom at 720mm, i.e. * 30, I would be able to read the text like if it was at 1 meter from me.
Then, how does equivalent focal works in this situation?
Or am I am totally mixing up different things?

My question may seem really dumb, but any enlightments would be really welcome to help me learn on this photography journey!

Best,

 

[dazzerfong]

The HX-60 actually has a lens with a real focal length of 4.3-129mm. Owing to the 'crop factor', its focal length equivalent to FF is 24-720mm. If you want it to have a similar field of view to an APS-C, it's 16mm-480mm.

I've never seen cameras besides those using FF, MFT, APS-C or MF sensors (or basically any interchangeable lens camera) will state their true focal length, and instead choose to state the focal length equivalent to FF.

With regards to optical zoom factor, your eyes don't work like a sensor. 30x just means that the longest focal length (720mm) is 30x your widest focal length (24mm). It does not necessarily mean you put a pair of 30x binoculars to your eyes.

 

[FrancoD]

As Dazzerfong has pointed out , you camera has a real focal length of 4.3 to 129mm, the 24-720 mm is already the equivalent in FF terms.

 

[FingerPainter]

Hi,

I am wondering about changing my actual camera and I would like to understand the concept of focal equivalent.
Indeed, my current camera is the hx60 which has a 1/2.3 sensor and a 24-720mm zoom lens.

As the first two posters have already pointed out, this is nor the actual focal length range of your camera. It is the FF equivalent. They may not have pointed our that the actual focal length range of the lens is printed right on the front of the lens.

...

So, in practice, let's say I am at 30 meters of a paper which has a 1 cm letter printed on it.
If I zoom at 720mm, i.e. * 30,

30x Zoom just means that the longest focal length is 30x the shortest focal length of the lens. A lens with a 20-600mm equivalent zoom range would be at 30 X when set to 600mm, but wouldn't give as much detail.

I would be able to read the text like if it was at 1 meter from me.

Not likely.

Then, how does equivalent focal works in this situation?

Convert it to FF equivalent focal length (FFEFL). Leave zoom ratio out of it.

Or am I am totally mixing up different things?

Maybe not totally, but partly.

My question may seem really dumb, but any enlightments would be really welcome to help me learn on this photography journey!

The smart thing you did was ask a question about something you didn't understand

Best,

At 30 m distance a 720mm FFEFT will frame a subject of 1m tall by 1.5m wide. If the type on the paper is 12 pt, then there would be about 10 lines of print and whitespace per inch x about 39 inches times about 6 pixel rows per character to read text (2340 pixel height) Your camera has about 1.5 the pixel height. If the lens is sharp enough and you don't have atmospheric distortion at 30m, you might be able to read the text.

 

[Planterz]

Adding to what's already been explained well by others...

Focal length of a lens is a property of the lens and only the lens. Different sensor sizes do not change the focal length.

Different sensor sizes however change the field of view through those lenses - tiny sensors have tiny field of views. The 1/2.3" sensor is pretty tiny, which allows tiny lenses with tiny focal lengths. That's how small cameras like your HX60 get crazy zoom ranges in small packages.

That narrow field of view is what gives you the "focal length" of 24-720mm, better described as effective, relative, or equivalent focal length (I like "effective"). These measurements are in relation to full frame sensor cameras, which have the same size (36x24mm) as classic 35mm film. Thus, if you had an actual (theoretical) 24-720mm lens on a full frame camera, your HX60 would have the same image framing. Comparison to full frame has become standard, so it's important to understand this relation even if you don't own a full frame camera, because you'll still be able to visualize focal lengths across systems with different crop factors, and saying "720mm" is kinda meaningless unless you have a reference point.

And speaking of crop factor, once again, everything is compared to full frame. A smaller sensor can be visualized as a "crop" of the full frame sensor, providing that narrower field of view through a lens. The Sony a6000 you mentioned in your other thread has an APS-C 1.5x crop factor, which means a 100mm lens will appear to have 1.5x the reach, equating to a field of view of a 150mm lens on a full frame camera. Canon's APS-C crop is 1.6x. Micro 4/3 is 2x, a 1" sensor has a 2.7x crop, and the 1/2.3" sensor of your HS60 has a 5.6x crop.

As mentioned by others, zoom is only in relation to itself. 24-720mm is a 30x zoom because 720/24=30. The human eye sees more like at a 40-45mm "full frame" focal length (hard to quantify exactly), so assume that a 720mm lens is very roughly a 16-18x magnification compared to your eyes.

 

[Tom Axford]

It may be helpful to look at the fundamental definition of focal length.

Consider the simple case of a pinhole camera. The lens of a normal camera is replaced by a pinhole:





The focal length is defined to be the image distance in the diagram above.

By simple geometry, we can work out the size of the the image of any object:

image size = object size x (image distance / object distance),

or

image size = object size x (focal length / object distance).

For example, if we photograph an object 2m tall at a distance of 10m from the pinhole, and using a focal length of 50mm, then the height of the image of that object will be (converting all lengths to mm): 2000 x 50 / 10000 = 10mm

If we double the focal length, then the image doubles in size. The image size is proportional to the focal length. It is sometimes said that the focal length determines the image magnification (which is defined simply as the image size / object size).

The same principles apply to a camera with a lens, but strictly speaking, the focal length is defined to be the image distance when the lens is focussed at infinity (it may differ slightly for closer focussing depending on the design of the lens).

I hope this is helpful.

 

[Michael Fryd]

Hi,

I am wondering about changing my actual camera and I would like to understand the concept of focal equivalent.
Indeed, my current camera is the hx60 which has a 1/2.3 sensor and a 24-720mm zoom lens.
So, if my computation are rights, when I am zoomed at maximum, i.e. 720mm, this would translate to 2595mm for an APS-C, up to my knowledge there is no such lens on the market (and I guess I understand why).

Computation is one thing but on the field is another.
So, in practice, let's say I am at 30 meters of a paper which has a 1 cm letter printed on it.
If I zoom at 720mm, i.e. * 30, I would be able to read the text like if it was at 1 meter from me.
Then, how does equivalent focal works in this situation?
Or am I am totally mixing up different things?

My question may seem really dumb, but any enlightments would be really welcome to help me learn on this photography journey!

Best,

"Focal Length" and "Equivalent Focal Length" are measures of completely different things.

"Focal Length" is a measure of a physical property of the lens. For simple lenses it is how far the lens would need to be from the sensor in order for something very far away to be in focus.

"Equivalent Focal Length" is a measure of the angle of view of the camera/lens combination. Personally, I think we would be better off expressing this in degrees, but that ship has sailed.

Angle of view is determined both by the physical focal length and the sensor size. "Equivalent Focal length" is simply the focal length needed on a full frame camera in order to get a particular angle of view.

For instance, a 50mm lens on a full frame camera has an angle of view of 46°. Therefore any camera/lens combination that has a 46° angle of view is said to have an angle of view of "50mm equivalent focal length".

For instance, a 25mm lens on a 2X crop body has a 46° angle of view, which is referred to as 50 mm equivalent focal length.

It can be confusing as "Focal Length" describes the lens, and "Equivalent Focal Length" describes the results of the lens on a particular body. Many are careless in the usage, and make it sound like the focal length of a lens changes when mounted on a crop body. This is not the case. The focal length remains the same, the angle of view changes.

 

[John Sheehy]

Hi,

I am wondering about changing my actual camera and I would like to understand the concept of focal equivalent.
Indeed, my current camera is the hx60 which has a 1/2.3 sensor and a 24-720mm zoom lens.
So, if my computation are rights, when I am zoomed at maximum, i.e. 720mm, this would translate to 2595mm for an APS-C, up to my knowledge there is no such lens on the market (and I guess I understand why).

If your zoom starts at "24mm", then that is areal 4.3mm already factored by the "FF crop factor" of about 5.58 for the 1/2.3" sensor. At max zoom, the focal length is 129 real mm.

 

[OldNikonFan2025]

So, if my computation are rights, when I am zoomed at maximum, i.e. 720mm, this would translate to 2595mm for an APS-C, up to my knowledge there is no such lens on the market (and I guess I understand why).

No need for me to expound on what others here have told you about this.

Computation is one thing but on the field is another.

Yep. So the question is what focal length is required on a larger format to match what you are getting on your smaller format, after cropping the larger format to match the FOV you are getting (in this instance, at 720mm "full frame equivalent")? The term I use here is "reach."

As it so happens I have done some comparisons in the past between the Panasonic ZS-70 and my D500 and repeatedly found that I could match the Panasonic using a good 280mm lens on my D500 or D850.

 

[John Sheehy]

So, if my computation are rights, when I am zoomed at maximum, i.e. 720mm, this would translate to 2595mm for an APS-C, up to my knowledge there is no such lens on the market (and I guess I understand why).

No need for me to expound on what others here have told you about this.

Computation is one thing but on the field is another.

Yep. So the question is what focal length is required on a larger format to match what you are getting on your smaller format, after cropping the larger format to match the FOV you are getting (in this instance, at 720mm "full frame equivalent")? The term I use here is "reach."

I don't like the term "reach", personally, as it implies a fictional change of distance. Used the way it is here only really means restriction of the AOV, which may give the viewfinder or LCD more of a telescopic function, but does not speak directly at all to the resolution of the subject. What good would an angle of view "equivalent" to 5000mm on a FF camera be if it only had 0.5MP resolution? Yes, it may help in the viewfinder to see what your subject is doing, but that is only because the wider angles of view have to shrink each object in the viewfinder more, obscuring detail which may in fact, actually be captured with a high pixel count.

As it so happens I have done some comparisons in the past between the Panasonic ZS-70 and my D500 and repeatedly found that I could match the Panasonic using a good 280mm lens on my D500 or D850.

The only area in which larger sensors can't easily follow is the "superzoom" range. Take the Nikon P1000, for example. To duplicate what it does in capturing pixels-on-subject from a given distance at "3000mm" FF-equivalent AOV, with the D500 pixel density, you would need a 1690mm lens. The speed of the lens would not be demanding, though, as you would need only f/25 to match pixel-level total light, diffraction, and DOF. To match AOV, you'd need 2000 real mm with the D500.

 

[OldNikonFan2025]

So, if my computation are rights, when I am zoomed at maximum, i.e. 720mm, this would translate to 2595mm for an APS-C, up to my knowledge there is no such lens on the market (and I guess I understand why).

No need for me to expound on what others here have told you about this.

Computation is one thing but on the field is another.

Yep. So the question is what focal length is required on a larger format to match what you are getting on your smaller format, after cropping the larger format to match the FOV you are getting (in this instance, at 720mm "full frame equivalent")? The term I use here is "reach."

I don't like the term "reach", personally, as it implies a fictional change of distance.

I don't see the implication you are inferring. When I extend my reach using a pole, the thing I was trying to reach didn't move.

Used the way it is here only really means restriction of the AOV, which may give the viewfinder or LCD more of a telescopic function, but does not speak directly at all to the resolution of the subject. What good would an angle of view "equivalent" to 5000mm on a FF camera be if it only had 0.5MP resolution? Yes, it may help in the viewfinder to see what your subject is doing, but that is only because the wider angles of view have to shrink each object in the viewfinder more, obscuring detail which may in fact, actually be captured with a high pixel count.

As it so happens I have done some comparisons in the past between the Panasonic ZS-70 and my D500 and repeatedly found that I could match the Panasonic using a good 280mm lens on my D500 or D850.

The only area in which larger sensors can't easily follow is the "superzoom" range. Take the Nikon P1000, for example. To duplicate what it does in capturing pixels-on-subject from a given distance at "3000mm" FF-equivalent AOV, with the D500 pixel density, you would need a 1690mm lens. The speed of the lens would not be demanding, though, as you would need only f/25 to match pixel-level total light, diffraction, and DOF. To match AOV, you'd need 2000 real mm with the D500.

It's not all just about pixel density. The diffraction hit to resolution also needs to be accounted for. FWIW, I didn't arrive at 280mm being the equivalent using formulas and a calculator, although I probably could have come close. I simply made visual comparisons of shots taken at different focal lengths with the D500 to what was the best I could do with the ZS-70, and I was actually able to do it much easier with the D500 and D850 (the D500 is a bit better in this regard owing to its greater viewfinder magnification). My guess is that the Nikon P1000 resolution could be matched at less than 1200mm on a D500, but the usefulness of such narrow AOVs using a handheld camera elude me.

 

[camera-user]

Hi,

I would like to thank all of you for your replies! This really helps me understanding this concept better!

The HX-60 actually has a lens with a real focal length of 4.3-129mm. Owing to the 'crop factor', its focal length equivalent to FF is 24-720mm. If you want it to have a similar field of view to an APS-C, it's 16mm-480mm.

I've never seen cameras besides those using FF, MFT, APS-C or MF sensors (or basically any interchangeable lens camera) will state their true focal length, and instead choose to state the focal length equivalent to FF.

With regards to optical zoom factor, your eyes don't work like a sensor. 30x just means that the longest focal length (720mm) is 30x your widest focal length (24mm). It does not necessarily mean you put a pair of 30x binoculars to your eyes.

So, it means that if I capture a scene with the following configurations:

1. 1/2.3 sensor with a 129mm focal lenght,
2. APS-C sensor with a 480mm focal lenght
3. and FF sensor with a 720mm focal lenght

I would get the exact same picture?

For zoom, I understand this is just a way to navigate the focal length of a lens, so it is just relative to this lens.
For example, a lens with a focal lenght ranging from 12mm to 36mm would offer a 3x zoom.
But this would really make sense to compare its zoom to another lens for another sensor.

 

[OldNikonFan2025]

Hi,

I would like to thank all of you for your replies! This really helps me understanding this concept better!

The HX-60 actually has a lens with a real focal length of 4.3-129mm. Owing to the 'crop factor', its focal length equivalent to FF is 24-720mm. If you want it to have a similar field of view to an APS-C, it's 16mm-480mm.

So, it means that if I capture a scene with the following configurations:

1. 1/2.3 sensor with a 129mm focal lenght,
2. APS-C sensor with a 480mm focal lenght
3. and FF sensor with a 720mm focal lenght

I would get the exact same picture?

You will get the same FOV looking through the viewfinder. The amount of resolution you will get depends on pixel density, diffraction, and the quality of the lens. The biggest difference in the pictures you will get will depend on the aperture you use at those focal lengths.

But this would really make sense to compare its zoom to another lens for another sensor.

What the compact cameras never mention is the equivalent aperture. There's a reason we say f/1.4, f/2, f/2.8, etcetera. The "f" refers to the focal length, and the "/" refers that focal length being divided by the diameter of the aperture, so if you have a 100mm f/4 lens than you have a 25mm diameter aperture (100 divided by 4 equals 25). This measures how much light is coming through the aperture, minus whatever light the glass elements are blocking. This also effects the DOF, so a smaller f-number means a larger aperture which means a shallower DOF.

As a practical matter, the f/6.3 at 129mm on an HX-60 means its aperture diameter is 20.5mm. If you divide 480mm by 20.5mm you come up with approximately f/24, and 720mm is f/35. When you double the f-number (so for example, going from f/2 to f/4) you also double the DOF, and that why you won't end up with the same picture using those different focal lengths on those formats.

 

[Michael Fryd]

Hi,

I would like to thank all of you for your replies! This really helps me understanding this concept better!

The HX-60 actually has a lens with a real focal length of 4.3-129mm. Owing to the 'crop factor', its focal length equivalent to FF is 24-720mm. If you want it to have a similar field of view to an APS-C, it's 16mm-480mm.

I've never seen cameras besides those using FF, MFT, APS-C or MF sensors (or basically any interchangeable lens camera) will state their true focal length, and instead choose to state the focal length equivalent to FF.

With regards to optical zoom factor, your eyes don't work like a sensor. 30x just means that the longest focal length (720mm) is 30x your widest focal length (24mm). It does not necessarily mean you put a pair of 30x binoculars to your eyes.

So, it means that if I capture a scene with the following configurations:

1. 1/2.3 sensor with a 129mm focal lenght,
2. APS-C sensor with a 480mm focal lenght
3. and FF sensor with a 720mm focal lenght

I would get the exact same picture?

For zoom, I understand this is just a way to navigate the focal length of a lens, so it is just relative to this lens.
For example, a lens with a focal lenght ranging from 12mm to 36mm would offer a 3x zoom.
But this would really make sense to compare its zoom to another lens for another sensor.

With the same subject, at the same angle of view, aperture diameter and shutter speed you get the "same" image. By that I mean, same depth of field, same diffraction issues, same motion blur, and even the same overall image noise. In other words, there is no inherent advantage to any of these from sensor size.

Obviously, if one camera has newer technology, more pixels, higher quality lenses, better processing, etc, than that camera will have an advantage (even if it has a smaller sensor).

The thing that trips many people up, is that they compare at different aperture diameters. Instead of using the same aperture diameter, they compare at the same ratio of focal length to diameter (i.e. the "f/stop").

Let's compare a full frame to a 2X crop body (Micro 4/3) as the math is easy with that comparison.

With the full frame you might be shooting at 50mm, f/8, 1/60 at ISO 400. With a 2X crop you would get the same results at 25mm, f/4, 1/60 and ISO 100.

The f/stop is simply the formula for computing the aperture diameter. "f/4" literally means that the aperture diameter is the focal length ("f") divided by 4. This both a 50mm lens at f/8, and a 25mm lens at f/4, have a 12.5mm aperture diameter.

The advantage of full frame is that you can get really large aperture diameters. A 50mm f/1.8 lens for my full frame camera is affordable and available at my local Best Buy. For a 2X crop, you would need a 25mm at f/0.9 to get that same 28mm aperture diameter.

Best Buy also carries full frame 50mm f/1.2 lenses. For a 2X crop you would need a 25mm f/0.6 in order to match that 42mm aperture diameter. That's not something carried by either Best Buy or my local camera store.

The advantage of full frame is that it has a wider shooting envelope. Generally you have the option of wider aperture diameters, and generally they can tolerate more total light captured. Unless you need one of those, the crop body can get the same results as the full frame.

A 6X crop factor "super zoom" with an f/2.8 lens will give you the same results as a full frame with the same angle of view at f/17.

 

[camera-user]

Hi,

I would like to thank all of you for your replies! This really helps me understanding this concept better!

The HX-60 actually has a lens with a real focal length of 4.3-129mm. Owing to the 'crop factor', its focal length equivalent to FF is 24-720mm. If you want it to have a similar field of view to an APS-C, it's 16mm-480mm.

So, it means that if I capture a scene with the following configurations:

1. 1/2.3 sensor with a 129mm focal lenght,
2. APS-C sensor with a 480mm focal lenght
3. and FF sensor with a 720mm focal lenght

I would get the exact same picture?

You will get the same FOV looking through the viewfinder. The amount of resolution you will get depends on pixel density, diffraction, and the quality of the lens. The biggest difference in the pictures you will get will depend on the aperture you use at those focal lengths.

But this would really make sense to compare its zoom to another lens for another sensor.

What the compact cameras never mention is the equivalent aperture. There's a reason we say f/1.4, f/2, f/2.8, etcetera. The "f" refers to the focal length, and the "/" refers that focal length being divided by the diameter of the aperture, so if you have a 100mm f/4 lens than you have a 25mm diameter aperture (100 divided by 4 equals 25). This measures how much light is coming through the aperture, minus whatever light the glass elements are blocking. This also effects the DOF, so a smaller f-number means a larger aperture which means a shallower DOF.

I just realized I forgot the "not" in the above sentence so: "But this would not really make sense to compare its zoom to another lens for another sensor."

As a practical matter, the f/6.3 at 129mm on an HX-60 means its aperture diameter is 20.5mm. If you divide 480mm by 20.5mm you come up with approximately f/24, and 720mm is f/35. When you double the f-number (so for example, going from f/2 to f/4) you also double the DOF, and that why you won't end up with the same picture using those different focal lengths on those formats.

I am not really familiar with the aperture.
Indeed, I rarely use the Aperture mode of my camera.
I only know this is what you use when you want to get bokeh effect.

So, in the above configuration I listed, I would get more detailed photos with the bigger sensors?

Let's take an example:
1. 1/2.3 sensor with a 129mm focal lenght and 6.3 aperture: 129/6.3 = 20.48
2. FF sensor with a 600mm focal lenght and 6.3 aperture: 600/6.3 = 95.24
So, I would get more details in the second configuration due to a broader DOF?

 

[Michael Fryd]

...

I am not really familiar with the aperture.
Indeed, I rarely use the Aperture mode of my camera.
I only know this is what you use when you want to get bokeh effect.

A scientist might tell you that there is only a single distance where objects are in focus. If your camera is focused at 10 feet, only objects exactly at 10 feet are in focus. Move just a hair further or closer, and the object is not technically in focus.

An engineer might point out that there is a range of distances where the focus is "good enough" for the human eye to perceive it as being in focus.

The exact range of the depth of field is dependent on many factors including viewing conditions and aperture diameter.

Roughly speaking, the aperture is a variable size in the optical path. When the hole is wider, it lets in more light but you get less depth of field. When the hole is smaller you get less light (resulting in a noisier image), but more depth of field.

A narrow depth of field can be aesthetically pleasing. With a head shot, it is common for a photographer to arrange for the head to be within the depth of field, but for the background to be out of focus.

The term "Bokeh" was coined to refer to the quality of the out of focus area. Depending on the lens, an out of focus white dot might look like a hexagon, a circle, or even a doughnut. If the out of focus area looks good, then we say the lens produced pleasing Bokeh.

So, in the above configuration I listed, I would get more detailed photos with the bigger sensors?
Let's take an example:
1. 1/2.3 sensor with a 129mm focal lenght and 6.3 aperture: 129/6.3 = 20.48
2. FF sensor with a 600mm focal lenght and 6.3 aperture: 600/6.3 = 95.24
So, I would get more details in the second configuration due to a broader DOF?

A 1/2.3 sensor has a crop factor of about 5.6X, If you want the same framing, you would need to compare 600mm on a full frame to 107mm on the 5.6X crop.

Assuming the same subject and shutter speeds, the Full Frame sensor will capture more light, and yield a less noisy image. The 5.6X crop sensor will yield more depth of field, but a noisier image. The 5.6X crop will also lose more sharpness due to diffraction issues.

If your subject fits within the depth of field, then the full frame will give a better result as you won't lose as much sharpness to diffraction issues as with the 5.6X crop.

If you stop the FF down to f/36, you will get the same results as the 5.6 crop body at f/6.3

 

[camera-user]

...

I am not really familiar with the aperture.
Indeed, I rarely use the Aperture mode of my camera.
I only know this is what you use when you want to get bokeh effect.

A scientist might tell you that there is only a single distance where objects are in focus. If your camera is focused at 10 feet, only objects exactly at 10 feet are in focus. Move just a hair further or closer, and the object is not technically in focus.

An engineer might point out that there is a range of distances where the focus is "good enough" for the human eye to perceive it as being in focus.

The exact range of the depth of field is dependent on many factors including viewing conditions and aperture diameter.

Roughly speaking, the aperture is a variable size in the optical path. When the hole is wider, it lets in more light but you get less depth of field. When the hole is smaller you get less light (resulting in a noisier image), but more depth of field.

A narrow depth of field can be aesthetically pleasing. With a head shot, it is common for a photographer to arrange for the head to be within the depth of field, but for the background to be out of focus.

The term "Bokeh" was coined to refer to the quality of the out of focus area. Depending on the lens, an out of focus white dot might look like a hexagon, a circle, or even a doughnut. If the out of focus area looks good, then we say the lens produced pleasing Bokeh.

OK! Thank you!
Using this animation:

Loading…

fr.wikipedia.org

I think I am now clearer!

So, the f/N denotes the aperture, with f constant, the bigger N the lower the aperture is.
And, as DOF = (2u²Nc)/f², with f constant the bigger N the deeper the DOF!
It then makes sense to have a small N for portrait, since we want to have small DOF to get Bokeh!

So, in the above configuration I listed, I would get more detailed photos with the bigger sensors?
Let's take an example:
1. 1/2.3 sensor with a 129mm focal lenght and 6.3 aperture: 129/6.3 = 20.48
2. FF sensor with a 600mm focal lenght and 6.3 aperture: 600/6.3 = 95.24
So, I would get more details in the second configuration due to a broader DOF?

A 1/2.3 sensor has a crop factor of about 5.6X, If you want the same framing, you would need to compare 600mm on a full frame to 107mm on the 5.6X crop.

Assuming the same subject and shutter speeds, the Full Frame sensor will capture more light, and yield a less noisy image. The 5.6X crop sensor will yield more depth of field, but a noisier image. The 5.6X crop will also lose more sharpness due to diffraction issues.

If your subject fits within the depth of field, then the full frame will give a better result as you won't lose as much sharpness to diffraction issues as with the 5.6X crop.

If you stop the FF down to f/36, you will get the same results as the 5.6 crop body at f/6.3

I understand for the bigger DOF with the crop sensor because (I fix u and c as constant to simplify):
1. 6.3/106² = .0005607
2. 6.3/600² = .0000175

So, I would indeed get a biffer DOF with the small sensor, but more details with the FF one, as I get more light in due having a bigger sensor.
Am I correct?

 

[Michael Fryd]

...

OK! Thank you!
Using this animation:
https://fr.wikipedia.org/wiki/Profo...ier:Profondeur_de_champ_selon_l'ouverture.gif
I think I am now clearer!

So, the f/N denotes the aperture, with f constant, the bigger N the lower the aperture is.

f/N is a fraction, so when N is larger the result is smaller. For instance 1/8 is smaller than 1/2.

To bring this to cameras, 50/8 is a smaller number than 50/4. Those are the aperture diameters for a 50mm lens at f/8 and at f/4.

And, as DOF = (2u²Nc)/f², with f constant the bigger N the deeper the DOF!
It then makes sense to have a small N for portrait, since we want to have small DOF to get Bokeh!

Sort of. First of all, I think you mean you want a small DoF in order to have the background out of focus. "Bokeh" is a term coined to refer to the quality of that out of focus area. Different lenses can render the out of focus areas differently.

Secondly, small DoF is not the only way to get the background out of focus. You can also move the subject further from the background.

One should be careful in thinking that DoF is directly dependent on f/stop. While that works if you are only dealing with a single sensor size, the reality is that it is the aperture diameter that's more important. F/2.8 may yield shallow DoF on a full frame, and deep DoF on a smart phone.

...

I understand for the bigger DOF with the crop sensor because (I fix u and c as constant to simplify):
1. 6.3/106² = .0005607
2. 6.3/600² = .0000175

So, I would indeed get a biffer DOF with the small sensor, but more details with the FF one, as I get more light in due having a bigger sensor.
Am I correct?

Sort of. but that's like saying you get a wider field of view with a full frame. Sure that's true if you shoot a full frame and a crop at the same focal length, but why would you choose to do that? If you want a normal angle of view (about 46°), you would choose a 50mm lens on a full frame and a 25mm lens on a 2X crop body. When you choose the appropriate focal length for the sensor size, you get the same angle of view.

Similarly, why would you shoot a crop body and a full frame at the same f/stop? You're going to get different results. If you are shooting your full frame at f/8, then shoot your 2X crop body at f/4. Then you get the same DoF, same diffraction, same overall image noise, and same sharpness (assuming similar quality lenses, processing and pixel counts).

.

Suppose you are shoot a 2X crop body with a 25mm lens and need to use f/5.6 in order to get enough DoF. You need to use 1/60 to stop motion, and you are getting good lightness at ISO 400. perhaps you want to reduce image noise, so you splurge and buy a full frame camera with an expensive 50mm f/1.2 lens.

The problem is that in order to get your needed depth of field, you need to at f/11. You still need 1/60 in order to stop motion, so you change the ISO to 1600.

Now you are getting the same results from your full frame as you got from your 2X crop body. In this situation there is no advantage to using the larger sensor.

Where the larger sensor helps, is in situations where you can tolerate a DoF shallower than what you can get on your crop body.

With a APS-C crop body, the difference is a little over one stop. At the same angle of view, you can generally find full frame lenses that open up about one stop wider in diameter than with an APS-C crop body. So if you can tolerate that shallower DoF, you can get an image with less noise.

,

But let's step back for a moment. Why the fixation on sharpness? You should be able to get reasonably sharp images with just about any modern 2X crop (or less) camera. If you are not getting sharp images, the issue may be technique, or a camera malfunction. It's probably not a limitation of the sensor size.

Keep in mind that content is almost always more important than sharpness. Increasing sharpness won't turn poor composition into a great image. There are lots of great images that are not sharp.

While there are certainly situation where one needs to absolutely maximize sharpness, they are not as common as some people think.

 

[OldNikonFan2025]

Using this animation:
https://fr.wikipedia.org/wiki/Profo...ier:Profondeur_de_champ_selon_l'ouverture.gif
I think I am now clearer!

So, the f/N denotes the aperture, with f constant, the bigger N the lower the aperture is.
And, as DOF = (2u²Nc)/f², with f constant the bigger N the deeper the DOF!
It then makes sense to have a small N for portrait, since we want to have small DOF to get Bokeh!

Bokeh is a lens characteristic. More blur is what you will get more of with a shallower DOF, and bokeh will become more noticeable.

I understand for the bigger DOF with the crop sensor because (I fix u and c as constant to simplify):
1. 6.3/106² = .0005607
2. 6.3/600² = .0000175

So, I would indeed get a biffer [deeper] DOF with the small sensor, but more details with the FF one, as I get more light in due having a bigger sensor.
Am I correct?

You are going in the right direction. Detail usually means more resolution, and that is primarily a function of optics and pixel density. More light gathered will give you more data, improving the S/N ratio enhances resolution, and less diffraction enhances resolution.

 

[OldNikonFan2025]

Where the larger sensor helps, is in situations where you can tolerate a DoF shallower than what you can get on your crop body.

Often the shallower DOF is desired rather than tolerated. OTOH, the most common beginner mistake moving to a larger format is not compensating for the narrower DOF at the same f-stop, and understanding equivalence is the answer to that.

But let's step back for a moment. Why the fixation on sharpness? You should be able to get reasonably sharp images with just about any modern 2X crop (or less) camera. If you are not getting sharp images, the issue may be technique, or a camera malfunction. It's probably not a limitation of the sensor size.

+1

Also, don't conflate sharpness with resolution.

Keep in mind that content is almost always more important than sharpness. Increasing sharpness won't turn poor composition into a great image. There are lots of great images that are not sharp.

While there are certainly situation where one needs to absolutely maximize sharpness, they are not as common as some people think.

+1

Resolution is often overrated. Display size and viewing distance dictate its necessity.