Cropping in post to mimic different focal length.

[Shane403]

Hello all,

I am shooting with a Sony A7CR and Fe 24mm F1.4. I am not good with the math side of things so I asked Chat GPT. Basically I am trying to figure out the cropping percentages of a photo to mimic 35mm, 50mm and 85mm focal lengths. My camera has a lot of megapixel so it is still producing 4K images after cropping over 100% in post. I am hoping for those of you who are very knowlegeable in this area and the calculation side of things can take a look at the info below and comment on how accurate and correct this is. If this is true, I might not have to buy all these lenses in differenct focal lengths. I understand that the background blur and/or compression looks will be different between cropping and using lenses with actual different focal lengths. Thank you in advance.

Here is what Chat GPT gave me:

To estimate how much cropping is needed to match different focal lengths on your Sony A7CR with the Sony FE 24mm f/1.4 lens, we need to compare the field of view (FOV) provided by each focal length relative to the camera's sensor.

Camera details:

• Sony A7CR sensor size: Full-frame (35mm equivalent)

Focal lengths compared:

• 24mm (your lens)
• 35mm
• 50mm
• 85mm

Calculation:

The crop factor for full-frame cameras is 1x, so the field of view is directly proportional to the focal length.

Equivalent cropping percentage:

• To get a 35mm view from 24mm, you need to crop approximately (24/35) ≈ 0.686, which is about 68.6% of the original image.
  • So, around 31.4% cropping of the original image to simulate a 35mm view.
• For 50mm, cropping ratio from 24mm:
  • (24/50) = 0.48, or 48% of the original image.
  • 52% cropping needed.
• For 85mm:
  • (24/85) ≈ 0.282, or 28.2% of the original image.
  • 71.8% cropping needed.

Summary:Target Focal LengthApproximate Crop Percentage of Original Image35mm31.4% (about one-third of the image)50mm52%85mm71.8%

In practical terms:

• Cropping from your 24mm image to simulate 35mm means cropping out roughly 68.6% of the frame.
• To simulate 50mm, crop about 52%.
• To simulate 85mm, crop about 72%.

Shane

A7CR + FE24F1.4

 

[Leonard Migliore]

Hello all,

I am shooting with a Sony A7CR and Fe 24mm F1.4. I am not good with the math side of things so I asked Chat GPT. Basically I am trying to figure out the cropping percentages of a photo to mimic 35mm, 50mm and 85mm focal lengths. My camera has a lot of megapixel so it is still producing 4K images after cropping over 100% in post. I am hoping for those of you who are very knowlegeable in this area and the calculation side of things can take a look at the info below and comment on how accurate and correct this is. If this is true, I might not have to buy all these lenses in differenct focal lengths. I understand that the background blur and/or compression looks will be different between cropping and using lenses with actual different focal lengths. Thank you in advance.

Here is what Chat GPT gave me:

The Chat GPT information appears correct to me.

I don't understand what you mean by "cropping over 100% in post". If you crop to 100% you're not cropping at all. If you crop by 100% you have nothing.

Looking up "4K image", I see that it's 8.3 MP. So you would crop your 60 MP camera to 37% of the original frame to get that. If you took the picture with a 24mm lens, it would have the same framing as if you had used a 65mm lens. If you crop down to simulate an 85mm lens, you would be left with less than 5 MP.

I do not recommend shooting wide and cropping. You would be better off using a phone. Some of them are very good.

 

[Tom Axford]

Hello all,

I am shooting with a Sony A7CR and Fe 24mm F1.4. I am not good with the math side of things so I asked Chat GPT. Basically I am trying to figure out the cropping percentages of a photo to mimic 35mm, 50mm and 85mm focal lengths. My camera has a lot of megapixel so it is still producing 4K images after cropping over 100% in post. I am hoping for those of you who are very knowlegeable in this area and the calculation side of things can take a look at the info below and comment on how accurate and correct this is. If this is true, I might not have to buy all these lenses in differenct focal lengths. I understand that the background blur and/or compression looks will be different between cropping and using lenses with actual different focal lengths. Thank you in advance.

Here is what Chat GPT gave me:

To estimate how much cropping is needed to match different focal lengths on your Sony A7CR with the Sony FE 24mm f/1.4 lens, we need to compare the field of view (FOV) provided by each focal length relative to the camera's sensor.

Camera details:

• Sony A7CR sensor size: Full-frame (35mm equivalent)

Focal lengths compared:

• 24mm (your lens)
• 35mm
• 50mm
• 85mm

Calculation:

The crop factor for full-frame cameras is 1x, so the field of view is directly proportional to the focal length.

Equivalent cropping percentage:

• To get a 35mm view from 24mm, you need to crop approximately (24/35) ≈ 0.686, which is about 68.6% of the original image.
  • So, around 31.4% cropping of the original image to simulate a 35mm view.
• For 50mm, cropping ratio from 24mm:
  • (24/50) = 0.48, or 48% of the original image.
  • 52% cropping needed.
• For 85mm:
  • (24/85) ≈ 0.282, or 28.2% of the original image.
  • 71.8% cropping needed.

Summary:Target Focal LengthApproximate Crop Percentage of Original Image35mm31.4% (about one-third of the image)50mm52%85mm71.8%

In practical terms:

• Cropping from your 24mm image to simulate 35mm means cropping out roughly 68.6% of the frame.
• To simulate 50mm, crop about 52%.
• To simulate 85mm, crop about 72%.

Shane

A7CR + FE24F1.4

The figures given by Chat GPT are essentially correct.

To go into more detail for the 85mm lens:

Its field of view is 28.2% of that of the 24mm lens. So the 9504x6336 pixel image needs to be cropped to 28.2% of this in both dimensions, i.e. 2680x1790 pixels approximately.

Something Chat GPT didn't mention is that the crop should always be in the centre of the image. If the crop is not centred, the perspective may be distorted (because a wide-angle lens such as the 24mm distorts the shape of some objects towards the corners of the image).

 

[FrancoD]

Never mind the numbers but, apart from emergencies, I don't see the point to have a camera like that to then crop that much .

 

[Michael Fryd]

The underlying rule is that with the same subject, at the same aperture diameter, angle of view and shutter speed, you get the same results.

This is independent of whether you got the angle of view by cropping, changing focal length, changing sensor size, etc.

.

Note: "aperture diameter" is the focal length divided by the f/stop. a 100mm lens at f/4 has an aperture diameter of 100mm/4, or 25mm.

.

Suppose you take a shot with a 50mm lens at f/4, and then you crop it down to match the angle of view from a 100mm lens. 50mm at f/4 is a 12.5mm aperture diameter.

The aperture diameter is not changed by cropping. If your angle of view now matches a 100mm lens with a12.5mm aperture diameter. A 12.5mm aperture diameter on a 100mm lens is f/8

Therefore, the results you get from cropping the 50mm f/4 shot, will match what you would get from a 100 mm lens at f/8 (at the same shutter speed), in terms of framing, depth of field, diffraction issues, motion blur and overall image noise.

.

By the way, cropping a 50mm on a full frame in order to match the angle of view of a 100mm lens, will get you the same results as mounting that 50mm lens on a 2X crop body.

A simple way to express this is that a 50mm lens at f/4 on a 2X crop body will yield the same results as a 100mm lens at f/8 on a full frame (assuming same shutter speed).

 

[New Wrycuda]

I am shooting with a Sony A7CR and Fe 24mm F1.4. I am not good with the math side of things so I asked Chat GPT. Basically I am trying to figure out the cropping percentages of a photo to mimic 35mm, 50mm and 85mm focal lengths. My camera has a lot of megapixel so it is still producing 4K images after cropping over 100% in post. I am hoping for those of you who are very knowlegeable in this area and the calculation side of things can take a look at the info below and comment on how accurate and correct this is. If this is true, I might not have to buy all these lenses in differenct focal lengths. I understand that the background blur and/or compression looks will be different between cropping and using lenses with actual different focal lengths. Thank you in advance.

Some cameras offer “Digital Zoom” which is the same as cropping after the event, but strictly limited to the centre of the frame. One of my cameras has a 35mm prime, and I find that 2x zoom (what you would refer to as 50% crop) is about as much as the camera can successfully support. It’s very handy for general photography.

Cropping after the event is best used for altering the composition or tidying up unwanted features. Cropping can then be applied to any part of the frame, with some restrictions when using wide-angle lenses due to what some refer to as distortion (I tend to refer to ‘perspective enhancement’). ;-)

It’s easy to get the numbers tangled when using percentages. Also be aware that when discussing overall image size, a crop that reduces one dimension by half also has the same effect on the other side, for example…

6000x4000 image = 24 Megapixels

3000x2000 image = 6 Megapixels.

Also note that most cameras are not capable of producing the theoretical resolution as calculated above, mainly because of the Bayer Filter that requires mathematical operations to render a viewable image, as well as the optical limitations of the lens.

Bayer Filter

 

[UncleVanya]

This is the key. The chat got percentages are for a single side. The effect is applied in both dimensions which reduces resolution a lot quicker than those numbers suggests.

 

[Guy Parsons]

I also chatted with ChatGPT about cropping a 24mm image to 85mm framing, here's what the nice AI thingy told me....



I think it's best to consider a suitable zoom or more prime lenses if thinking that you need only a single 24mm lens.

 

[Bob A L]

Leica seems to think it is fine. They sell an expensive camera, Leica Q3 and Q3 43 that does just this. It has fixed focal length and uses image cropping in camera to achieve different focal lengths. It works well with my Sony 61mp camera also.

 

[New Wrycuda]

Leica seems to think it is fine. They sell an expensive camera, Leica Q3 and Q3 43 that does just this. It has fixed focal length and uses image cropping in camera to achieve different focal lengths. It works well with my Sony 61mp camera also.

The Sony a5100 has a special zoom lever just near the shutter button. The kit lens (E-PZ 16-50mm) is a “Power Zoom”, as suggested by the name, and the lever will also apply digital zoom to a degree specified in the menu.

At the moment, there’s a very good non-zoom 35mm lens fitted to the camera, and I have restricted digital zoom to 2x.

Beginners should note that this example was taken with the camera set to “Superior Auto”…


No zoom.


2x digital zoom. Still 6000x4000 due to in-camera processing and interpolation.

 

[Martin_99]

Hello all,

I am shooting with a Sony A7CR and Fe 24mm F1.4. I am not good with the math side of things so I asked Chat GPT. Basically I am trying to figure out the cropping percentages of a photo to mimic 35mm, 50mm and 85mm focal lengths. My camera has a lot of megapixel so it is still producing 4K images after cropping over 100% in post. I am hoping for those of you who are very knowlegeable in this area and the calculation side of things can take a look at the info below and comment on how accurate and correct this is. If this is true, I might not have to buy all these lenses in differenct focal lengths. I understand that the background blur and/or compression looks will be different between cropping and using lenses with actual different focal lengths. Thank you in advance.

Each further crop degrade image quality a bit. You can imagine it as you would shoot with smaller sensor format. If you crop from 24mm:

35mm - APSC crop

50mm - m4/3 crop

85mm - 1" sensor crop?

At that point there could be a question why to actually buy an expensive fulframe camera.

Here is what Chat GPT gave me:

To estimate how much cropping is needed to match different focal lengths on your Sony A7CR with the Sony FE 24mm f/1.4 lens, we need to compare the field of view (FOV) provided by each focal length relative to the camera's sensor.

Camera details:

• Sony A7CR sensor size: Full-frame (35mm equivalent)

Focal lengths compared:

• 24mm (your lens)
• 35mm
• 50mm
• 85mm

Calculation:

The crop factor for full-frame cameras is 1x, so the field of view is directly proportional to the focal length.

Equivalent cropping percentage:

• To get a 35mm view from 24mm, you need to crop approximately (24/35) ≈ 0.686, which is about 68.6% of the original image.
  • So, around 31.4% cropping of the original image to simulate a 35mm view.

Seems not correct to me. If you make APSC crop (1.53 ratio) from 61Mpx sensor, you will get 26Mpx image, it's roughly 36mm focal length. So it will be around 40% of original image already at 35mm.

• For 50mm, cropping ratio from 24mm:
  • (24/50) = 0.48, or 48% of the original image.
  • 52% cropping needed.
• For 85mm:
  • (24/85) ≈ 0.282, or 28.2% of the original image.
  • 71.8% cropping needed.

Summary:Target Focal LengthApproximate Crop Percentage of Original Image35mm31.4% (about one-third of the image)50mm52%85mm71.8%

In practical terms:

• Cropping from your 24mm image to simulate 35mm means cropping out roughly 68.6% of the frame.
• To simulate 50mm, crop about 52%.
• To simulate 85mm, crop about 72%.

Shane

A7CR + FE24F1.4

 

[J A C S]

I also chatted with ChatGPT about cropping a 24mm image to 85mm framing, here's what the nice AI thingy told me....

Right. The way it is formulated in the OP is essentially wrong. No distinction is made between linear and area ratios.

 

[Bob]

Whatever focal length I happen to be using, I still crop the image in post in order to get the right composition.

Why do you need to crop to a specific focal length rather than cropping to a desired composition? Sounds like the tail is wagging the dog unless I'm missing something.

Even if you were to crop a 35mm lens to precisely mimic an 85mm lens, what if you want to change the aspect ratio to, say, 1:1 (i.e. square)? Your calculations would be less useful in that scenario.

 

[John Sheehy]

In practical terms:

• Cropping from your 24mm image to simulate 35mm means cropping out roughly 68.6% of the frame.
• To simulate 50mm, crop about 52%.
• To simulate 85mm, crop about 72%.

There is more to consider. You need to consider the number of pixels that remain. You also need to consider how much cropping you can get away with in light of the effects of lens aberration and diffraction.