Crop sensor effect on apertures

[Peter 13]

There is FOV difference but no DOF difference. DOF will be different ONLY if you try to make FOV the same by: 1) change distance or 2) change focal length. This has been repeated by so many people so many times I really don't know why there are still people who can't get it.

When you crop, and display/print the same size, the apparent softness increases (so called COC - a term that I hate). This decreases the DOF.

I really don't know why there are still people who can't get it.

Here is an example (taken from Bob Atkins DOF but any other DOF calculator will give you the same):

FF, FL=50mm, distance to object = 2m, f/2. DOF = 0.19m
crop, FL=50mm, distance to object = 2m, f/2. DOF = 0.12m

 

[Great Bustard]

This is the myth we are trying to dispell. 200 f2 on crop gives you.....200 f2 with a cropped image. The FOV matches a 320mm. To get the same framing as you would an uncropped image you back up, and THAT changes the DOF.

Unless a photographer is focal length limited, I'm not sure what the sense is in comparing the DOFs between photos between two systems that have different perspectives or different framing.

For example, would a FF shooter intentionally use a 200 / 2L IS and crop to 320mm, or use a 300 / 2.8L IS, unless the longest lens they had was 200mm?

Furthermore, I'm not sure what sense there is in comparing photos with different dimensions. For example, why would we compare an 8 x 12 inch print on 1.6x with a 13 x 19 inch print on FF?

More directly, DOF is a function of several variables. These variables are:

• subject-camera distance (perspective)
• framing (AOV)
• aperture diameter (focal length divided by f-ratio)
• display dimensions
• viewing distance
• visual acuity

Furthermore, if the sensors have the same efficiency, they will also have the same noise for the same shutter speed (despite the ISO differential).

That is, noise is a function of the following variables:

• scene luminance
• distance from scene (perspective)
• aperture diameter (focal length divided by f-ratio)
• shutter speed
• sensor efficiency (which, for some sensors, actually increases with higher ISOs)

The first four factors determine how much light falls on the sensor, and the last factor determines how much of that light is recorded, and how much additional noise is introduced.

Basically, in terms of how the equipment is actually used, and in terms of the visual properties of the final photo , it makes the most sense to say that 200mm f/2 on 1.6x is equivalent to 300mm f/3.2 on FF.

 

[Keith Z Leonard]

We always get back to this point, in practice you are correct, if you assume the same framing of the photo then you can think of it as having the DOF of a different aperture, the problem is that this assumption isn't stated most of the time and it can easily become confusing to the reader/beginner who thinks the f2.8 lens they just bought it's f2.8 on their camera.

 

[Keith Z Leonard]

I fight your web post with this web post...

http://www.have-camera-will-travel.com/field_reports/full_frame_vs_crop_sensor_-.html

I suppose I would separate circle of confusion artifacts from depth of field as you are talking about magnification blur, which will happen regardless of whether or not you are in the plane of focus, yeah?

btw, this is all just fun tech talk, hope you know I'm not taking it TOO seriously or anything....

 

[Great Bustard]

We always get back to this point, in practice you are correct, if you assume the same framing of the photo then you can think of it as having the DOF of a different aperture, the problem is that this assumption isn't stated most of the time and it can easily become confusing to the reader/beginner who thinks the f2.8 lens they just bought it's f2.8 on their camera.

The point of my post was that the natural assumption when comparing systems is to compare with the same perspective, framing, and display dimensions. What reason is there to compare otherwise, except when one is focal length or magnification limited?

In addition, it's often also important to compare at the same shutter speed, but this is a less stringent condition, since it is common for the shutter speed to be fast enough at base ISO where motion blur and/or camera shake are not an issue at the desired aperture.

But, no, a 200 / 2 lens isn't 200 / 2 on FF and 320 / 3.2 on 1.6x -- neither the focal length nor the f-ratio change simply because we use the lens on a different body.

But the effect of the focal length and f-ratio do change, so, yes, it makes sense to say that 200 / 2 on FF is equivalent to 320 / 3.2 on 1.6x.

 

[Peter 13]

I fight your web post with this web post...

http://www.have-camera-will-travel.com/field_reports/full_frame_vs_crop_sensor_-.html

If a guy on the web cannot see a difference, then there must be no difference, right?

EDIT: It is very clear there that the picture on the right has less DOF: about 1.6 cm vs. 2 cm. The confusion comes from the fact that the area withthe smaller DOF for this particular shot look larger on your screen.

Here is a quote from wikipedia:

If pictures are taken from the same distance using the same f-number, and the final images are the same size, the smaller format has less DOF.

 

[PRPhotos]

When you crop, and display/print the same size, the apparent softness increases This decreases the DOF.

Prove It !

Take any photo you want and crop it and post both the original and the cropped back here and show us the difference in DOF....

 

[Keith Z Leonard]

Right, but I disagree that it's a natural assumption, as is evidenced by the number of people confused by the topic. We should just be more explicit when we post, or really, there should be a sticky or something, though those will probably get ignored as well.

 

[Keith Z Leonard]

ha ha, wikipedia quote for the win! Maybe I should just edit that, then my argument will be more valid right? .....right?

 

[Great Bustard]

When you crop, and display/print the same size, the apparent softness increases This decreases the DOF.

Prove It !

Take any photo you want and crop it and post both the original and the cropped back here and show us the difference in DOF....

Here ya go:

http://forums.dpreview.com/forums/read.asp?forum=1029&message=23910313

When viewing, keep in mind:

http://www.josephjamesphotography.com/equivalence/#dof

On the other hand, the DOF formulas do not include how closely we scrutinize and image. In other words, two images might have the same DOF per the mathematical formulas, but if we scrutinize one image more closely than another (perhaps it is more interesting, for example), then the DOFs may appear different:

Scrutinizing one image more critically than another has the same effect as looking at that image with a higher visual acuity than the another.

However, for two images of the same scene displayed at the same size and viewed from the same distance that have the same computed DOF, then whatever the subjective impression of the DOF is for one image, it will be the same for the other image (although, as discussed above, it's easy to confuse "blurry" with "less DOF").

 

[Great Bustard]

Right, but I disagree that it's a natural assumption, as is evidenced by the number of people confused by the topic.

Fair point. But I would be interested to know why someone would think that comparing systems with different perspectives, different framings, and/or different display sizes.

We should just be more explicit when we post, or really, there should be a sticky or something, though those will probably get ignored as well.

DPR doesn't support stickies, but, yes, it's important to spell out the conditions of a comparison. However, the difficulty is that in, say, a thread like this, unless the conditions are spelled out in every post, the first time they're not spelled out, there will be at least one person who posts as if they were never mentioned. Actually, no need to even invoke that problem -- my experience is that there are more than enough that don't read what's written even when it is posted multiple times.

 

[PhillipA]

Phillip A has it correct, it does not change the properties of the lens, but since to keep the same framing people move back more, that changes the apparent DOF. The physics don't change by magic, the use changes.

... I was starting to think there was no-one out there that understands basic physics ;-)

--

Judge: ' This image may be better in black and white - perhaps even just black! '

 

[PRPhotos]

However, for two images of the same scene displayed at the same size and viewed from the same distance that have the same computed DOF, then whatever the subjective impression of the DOF is for one image, it will be the same for the other image (although, as discussed above, it's easy to confuse "blurry" with "less DOF").

So for example,

In PP you take an original picture and you do a 100% crop and a 200% crop and post all 3 sized to the same dimensions.

Are you actually seeing a true change in DOF or a "percieved" change in DOF do to magnifying the blur that is already present in the original shot ?

 

[Peter 13]

So for example,

In PP you take an original picture and you do a 100% crop and a 200% crop and post all 3 sized to the same dimensions.

Are you actually seeing a true change in DOF or a "percieved" change in DOF do to magnifying the blur that is already present in the original shot ?

Yes, that is why people pixel peep at 100%.

 

[Bogdan_M]

However, for two images of the same scene displayed at the same size and viewed from the same distance that have the same computed DOF, then whatever the subjective impression of the DOF is for one image, it will be the same for the other image (although, as discussed above, it's easy to confuse "blurry" with "less DOF").

So for example,

In PP you take an original picture and you do a 100% crop and a 200% crop and post all 3 sized to the same dimensions.

Are you actually seeing a true change in DOF or a "percieved" change in DOF do to magnifying the blur that is already present in the original shot ?

The definition of DOF takes into the account the magnification, therefore this is, technically, a "true" change in DOF (you will change the magnification, therefore the value of the CoC, therefore the value of the DOF).

--
Kind regards,

Bogdan

 

[Peter 13]

ha ha, wikipedia quote for the win! Maybe I should just edit that, then my argument will be more valid right? .....right?

Why do not you do it?

Then you should write to all people having DOF calculators and tell them to stop asking the stupid question about the camera format. As you say, it is not needed.

Then you should write to the autors of all photography/optics books asking them to fix their mistakes.

I am giving you references only because you refuce to understand the concept. It is very simple. You somehow define an acceptable blur raduis for a given print size. When you crop, and magnify to the same size, that radius increases by the crop factor. Then a smaller interval fits into your acceptable range.

 

[Keith Z Leonard]

I believe I understand the concept of how circle of confusion affects perceived image sharpness, which is not the same as depth of field. If the image is already blurred at that point, then you are getting your measurement of the depth of field wrong in the first place. These should always be measured at the pixel level, so as to have consistent results. By your logic if I resize my image down using a sharpening filter I get increased depth of field from the lens. That's just not the case.

 

[Bogdan_M]

"Thus, that image must be enlarged more to produce the final photograph. This, by itself, would yield less DOF."

It's all here:
http://www.dofmaster.com/dof_dslr.html

--
Kind regards,

Bogdan

 

[Keith Z Leonard]

Interesting, I just posted a response to Peter where I argue the opposite, perhaps I am wrong in that I would not take magnification to be a part of depth of field. So from now on I will shoot at f1.4 with my prime lens and resize to 1/2 size to "increase" my depth of field...seems silly to me.

I would have measured depth of field at the pixel level of the original image, so I suppose I'm wrong if the definition contains circle of confusion from magnification.

 

[Bogdan_M]

It's not his logic, really, it's how DOF is calculated.

Calculating things at the pixel level will never yeld consistent results, because cameras have different pixel counts.

Consistency is achieved by normalizing the results, that is choosing the same output size (as for a print) and the same viewing distance. Really, read this, it's the reference in this field.

http://www.dofmaster.com/

I believe I understand the concept of how circle of confusion affects perceived image sharpness, which is not the same as depth of field. If the image is already blurred at that point, then you are getting your measurement of the depth of field wrong in the first place. These should always be measured at the pixel level, so as to have consistent results. By your logic if I resize my image down using a sharpening filter I get increased depth of field from the lens. That's just not the case.

--
Kind regards,

Bogdan