Do we calculate Aperture to when using FF lenses on Crop Bodies?

jack002 · 12-13-2019, 09:44 AM - **1 Like**

Lenses all have one focal length, they never change.
Crop sensors crop images.
Take a photo with a 18mm on a FF, now crop it way down, that is the cropped photo, no change in focal length or aperture
So look out a window and see outside, now hold a toilet paper tube up to one eye, you are looking at a cropped full frame image.

[edit]

You might have come upon a tip somewhere about "calculating crop factor" or how to calculate what focal len your lens will be on a cropped sensor camera.
Those are technically false information and the real answer is more complex.
You do know that images appear differently on different focal lengths? The wide angle view bends lines that are straight and long lenses flatten everything out? Moved from one size sensor to another, this is still true. So if you were told a 50mm looks like 70mm in a crop sensor, try taking a test shot of someone's face with a 70mm on a FF and a 50 on a cropped camera and compare. The one shot with a 50mm, the person's nose will look bigger, the lines going into the distance will converge sooner, the lens is different, you will not see the same image in both, even though you will see about the same items at the same distance.

Last edited by jack002; 12-13-2019 at 10:17 AM.

Wasp · 12-13-2019, 11:20 AM - **2 Likes**

The crop is 1.5. Multiply focal length by crop factor, i.e. 77x1.5=115.5mm. Make that 120mm for simplicity.

Multiply aperture by crop factor, i.e 1.8x1.5=2.7. Make that 2.8 for simplicity.

In short, the 77mm f/1.8 on APS-C will give the same angle of view and depth of field as a 120mm f/2.8 on full frame. Pentax made such a lens, once upon a time.

SMC Pentax-M 120mm F2.8 Reviews - M Prime Lenses - Pentax Lens Reviews & Lens Database

bdery · 12-13-2019, 11:28 AM - **2 Likes**

Originally posted by Wasp

The crop is 1.5. Multiply focal length by crop factor, i.e. 77x1.5=115.5mm. Make that 120mm for simplicity.

Multiply aperture by crop factor, i.e 1.8x1.5=2.7. Make that 2.8 for simplicity.

In short, the 77mm f/1.8 on APS-C will give the same angle of view and depth of field as a 120mm f/2.8 on full frame.

Ohhh, that's a risky way of tackling this question!

Putting aside the fact that these are pretty big approximations, this line of reasoning supposes that a person wants to create the exact same image with a FF and an APS-C camera. That's a messy way to reason, because the equivalence will not hold for all apertures, subject distance, focal lengths, etc.

If you want to create a general rule this way, you'd have to take into account the pixel size since it influences the circle of confusion, which then influences the perceived DOF.

I repeat, it's a mess, and there are no general rules when going down this road.

A crop sensor simply removes the edges of an image, leaving the rest unchanged. It's much, MUCH simpler to reason that way. Any other trail will get people lost.

Wasp · 12-13-2019, 01:49 PM

Oversimplifying things doesn't help much. Smaller sensor means less depth of field. As long as you keep that in mind, you will be alright. It can be real deal breaker for those looking at Micro Four Thirds, but let's no dip into that can of worms, hehehehe.

The crop factor is a good way to compare things. It may not hold for every use case out there but it is still the easiest way to explain things. Unfortunately, it has turned into a bit of an obsession for the internet commentariat. Unless one owns, say, both a K1 and a K3, there is little reason for all the fuss.

If you have, as I do, a Kx with a Rokinon 85mm f/1.4, you get a certain kind of picture. Doing math with crop factors doesn't change my pictures, but using it has given me a feeling for the results that are possible. Using said oversimplified math I do know that if I ever get a K1, I will need a Rokinon 135mm f/2 to go with it to achive the same look and feel. It won't be exactly the same, but it is about the closest match that exists in the real world.

jack002 · 12-13-2019, 02:20 PM

Really? A sensor size affects the depth of field? So a f2.0 lens at f2.0 on different cameras has a different DOF? I never heard that. The field of view changes, yes, that's all. *confused*

swanlefitte · 12-13-2019, 02:32 PM - **2 Likes**

Originally posted by jack002

Really? A sensor size affects the depth of field? So a f2.0 lens at f2.0 on different cameras has a different DOF? I never heard that. The field of view changes, yes, that's all. *confused*

Again the k1 in crop or full mode records the exact same pixels. Yet
K1 crop 40mm f2.8 at 10m has dof=7.54m and in full mode dof=14.6m. This is because to print the less pixels of the crop to 8x10 you have to enlarge more than the greater number of pixels of the full sensor. Thus also enlarging any fuzziness so it can be perceived.

Sandy Hancock · 12-13-2019, 02:36 PM - **2 Likes**

With a given f-stop on different sensor sizes, depth of field only changes if you move backwards or forwards to get the same framing.

So back to Eddy's original question - your FA77 at a given aperture will give exactly the same exposure on a full frame, APS-C, micro 4/3 or Q body. If you don't change your position relative the to the subject, the depth of acceptable focus doesn't change either - only your angle of view.

bertwert · 12-13-2019, 02:40 PM

Originally posted by Wasp

Smaller sensor means less depth of field.

So, in a hypothetical situation, we take two identical lenses, say a 50mm lens set at f/1.4 and we put one on a camera with a FF sensor, and one on a camera with an APS-C. Then take two photos from the same spot of the same subject, let's say 5 metres away, and both cameras are focused at exactly 5 metres. The resulting images will have different DoF? How does this work. If this lens is projecting the same image (which it is) on each sensor, how come the properties of the image are different (apart from the area of the image we are using, obviously the APS-C sensor will capture an image cropped from a bigger image). I'm not saying that you're wrong, I'm just failing to understand

Wasp · 12-13-2019, 02:52 PM

One (perhaps oversimplified) way of looking at it is that by using a smaller part of the image circle, you have in effect stopped down the aperture - therefore you get more DOF.

swanlefitte · 12-13-2019, 02:54 PM - **1 Like**

Originally posted by Sandy Hancock

With a given f-stop on different sensor sizes, depth of field only changes if you move backwards or forwards to get the same framing.

Check on a dof calculator. Same distance, same fl, same fstop different sensor size gives different dof.

bertwert · 12-13-2019, 03:09 PM

Originally posted by swanlefitte

Check on a dof calculator. Same distance, same fl, same fstop different sensor size gives different dof.

But why?

swanlefitte · 12-13-2019, 03:17 PM - **1 Like**

Originally posted by bertwert

But why?

See post 21.

Wasp · 12-13-2019, 03:24 PM - **1 Like**

Originally posted by bertwert

But why?

Because mathematics.

This might help:

https://damienfournier.co/dof-the-simplified-formula-to-understand-dof/

bertwert · 12-13-2019, 03:30 PM

So if I had a K-1, and took a photo in APS-C crop mode, then took a photo in FF mode (from the exact same place with the exact same settings), then cropped the FF photo to be the same as the APS-C crop photo, the two photos would be different? I just can't wrap my head around it...

swanlefitte · 12-13-2019, 03:41 PM - **1 Like**

Originally posted by bertwert

So if I had a K-1, and took a photo in APS-C crop mode, then took a photo in FF mode (from the exact same place with the exact same settings), then cropped the FF photo to be the same as the APS-C crop photo, the two photos would be different? I just can't wrap my head around it...

Cropping the full photo and then enlarging it will change dof. Also printing larger or smaller than 8x10 will change Dof. And viewing from a closer or farther distance will change dof. Can you see when a point looks fuzzy or not is dof. Close up it's fuzzy at distance it's not. Large it's fuzzy and small it isn't. Dof is about perception of focus, not true focus.

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12-13-2019, 09:44 AM - 1 Like	#16
jack002 Veteran Member Join Date: Sep 2014 Location: Missouri Photos: Albums Posts: 727	Lenses all have one focal length, they never change. Crop sensors crop images. Take a photo with a 18mm on a FF, now crop it way down, that is the cropped photo, no change in focal length or aperture So look out a window and see outside, now hold a toilet paper tube up to one eye, you are looking at a cropped full frame image. [edit] You might have come upon a tip somewhere about "calculating crop factor" or how to calculate what focal len your lens will be on a cropped sensor camera. Those are technically false information and the real answer is more complex. You do know that images appear differently on different focal lengths? The wide angle view bends lines that are straight and long lenses flatten everything out? Moved from one size sensor to another, this is still true. So if you were told a 50mm looks like 70mm in a crop sensor, try taking a test shot of someone's face with a 70mm on a FF and a 50 on a cropped camera and compare. The one shot with a 50mm, the person's nose will look bigger, the lines going into the distance will converge sooner, the lens is different, you will not see the same image in both, even though you will see about the same items at the same distance. Last edited by jack002; 12-13-2019 at 10:17 AM.

12-13-2019, 11:20 AM - 2 Likes	#17
Wasp Pentaxian Join Date: Mar 2017 Location: Pretoria Photos: Gallery Posts: 4,659	The crop is 1.5. Multiply focal length by crop factor, i.e. 77x1.5=115.5mm. Make that 120mm for simplicity. Multiply aperture by crop factor, i.e 1.8x1.5=2.7. Make that 2.8 for simplicity. In short, the 77mm f/1.8 on APS-C will give the same angle of view and depth of field as a 120mm f/2.8 on full frame. Pentax made such a lens, once upon a time. SMC Pentax-M 120mm F2.8 Reviews - M Prime Lenses - Pentax Lens Reviews & Lens Database

12-13-2019, 11:28 AM - 2 Likes	#18
bdery Pentaxian Join Date: Mar 2008 Location: Quebec city, Canada Photos: Gallery Posts: 9,357	Originally posted by Wasp The crop is 1.5. Multiply focal length by crop factor, i.e. 77x1.5=115.5mm. Make that 120mm for simplicity. Multiply aperture by crop factor, i.e 1.8x1.5=2.7. Make that 2.8 for simplicity. In short, the 77mm f/1.8 on APS-C will give the same angle of view and depth of field as a 120mm f/2.8 on full frame. Ohhh, that's a risky way of tackling this question! Putting aside the fact that these are pretty big approximations, this line of reasoning supposes that a person wants to create the exact same *image* with a FF and an APS-C camera. That's a messy way to reason, because the equivalence will not hold for all apertures, subject distance, focal lengths, etc. If you want to create a general rule this way, you'd have to take into account the pixel size since it influences the circle of confusion, which then influences the perceived DOF. I repeat, it's a mess, and there are no general rules when going down this road. A crop sensor simply removes the edges of an image, leaving the rest unchanged. It's much, MUCH simpler to reason that way. Any other trail will get people lost.

12-13-2019, 01:49 PM	#19
Wasp Pentaxian Join Date: Mar 2017 Location: Pretoria Photos: Gallery Posts: 4,659	Oversimplifying things doesn't help much. Smaller sensor means less depth of field. As long as you keep that in mind, you will be alright. It can be real deal breaker for those looking at Micro Four Thirds, but let's no dip into that can of worms, hehehehe. The crop factor is a good way to compare things. It may not hold for every use case out there but it is still the easiest way to explain things. Unfortunately, it has turned into a bit of an obsession for the internet commentariat. Unless one owns, say, both a K1 and a K3, there is little reason for all the fuss. If you have, as I do, a Kx with a Rokinon 85mm f/1.4, you get a certain kind of picture. Doing math with crop factors doesn't change my pictures, but using it has given me a feeling for the results that are possible. Using said oversimplified math I do know that if I ever get a K1, I will need a Rokinon 135mm f/2 to go with it to achive the same look and feel. It won't be exactly the same, but it is about the closest match that exists in the real world.

12-13-2019, 02:20 PM	#20
jack002 Veteran Member Join Date: Sep 2014 Location: Missouri Photos: Albums Posts: 727	Really? A sensor size affects the depth of field? So a f2.0 lens at f2.0 on different cameras has a different DOF? I never heard that. The field of view changes, yes, that's all. confused

12-13-2019, 02:32 PM - 2 Likes	#21
swanlefitte Pentaxian Join Date: May 2015 Location: Minneapolis Photos: Gallery Posts: 4,068	Originally posted by jack002 Really? A sensor size affects the depth of field? So a f2.0 lens at f2.0 on different cameras has a different DOF? I never heard that. The field of view changes, yes, that's all. confused Again the k1 in crop or full mode records the exact same pixels. Yet K1 crop 40mm f2.8 at 10m has dof=7.54m and in full mode dof=14.6m. This is because to print the less pixels of the crop to 8x10 you have to enlarge more than the greater number of pixels of the full sensor. Thus also enlarging any fuzziness so it can be perceived.

12-13-2019, 02:36 PM - 2 Likes	#22
Sandy Hancock Moderator Site Supporter Join Date: Feb 2012 Location: Adelaide Hills, South Australia Photos: Gallery \| Albums Posts: 11,269	With a given f-stop on different sensor sizes, depth of field only changes if you move backwards or forwards to get the same framing. So back to Eddy's original question - your FA77 at a given aperture will give exactly the same exposure on a full frame, APS-C, micro 4/3 or Q body. *If you don't change your position relative the to the subject*, the depth of acceptable focus doesn't change either - only your angle of view.

12-13-2019, 02:40 PM	#23
bertwert Veteran Member Join Date: Jun 2015 Location: Golden, BC Posts: 15,173	Originally posted by Wasp Smaller sensor means less depth of field. So, in a hypothetical situation, we take two identical lenses, say a 50mm lens set at f/1.4 and we put one on a camera with a FF sensor, and one on a camera with an APS-C. Then take two photos from the same spot of the same subject, let's say 5 metres away, and both cameras are focused at exactly 5 metres. The resulting images will have different DoF? How does this work. If this lens is projecting the same image (which it is) on each sensor, how come the properties of the image are different (apart from the area of the image we are using, obviously the APS-C sensor will capture an image cropped from a bigger image). I'm not saying that you're wrong, I'm just failing to understand

12-13-2019, 02:52 PM	#24
Wasp Pentaxian Join Date: Mar 2017 Location: Pretoria Photos: Gallery Posts: 4,659	One (perhaps oversimplified) way of looking at it is that by using a smaller part of the image circle, you have in effect stopped down the aperture - therefore you get more DOF.

12-13-2019, 02:54 PM - 1 Like	#25
swanlefitte Pentaxian Join Date: May 2015 Location: Minneapolis Photos: Gallery Posts: 4,068	Originally posted by Sandy Hancock With a given f-stop on different sensor sizes, depth of field only changes if you move backwards or forwards to get the same framing. Check on a dof calculator. Same distance, same fl, same fstop different sensor size gives different dof.

12-13-2019, 03:09 PM	#26
bertwert Veteran Member Join Date: Jun 2015 Location: Golden, BC Posts: 15,173	Originally posted by swanlefitte Check on a dof calculator. Same distance, same fl, same fstop different sensor size gives different dof. But why?

12-13-2019, 03:17 PM - 1 Like	#27
swanlefitte Pentaxian Join Date: May 2015 Location: Minneapolis Photos: Gallery Posts: 4,068	Originally posted by bertwert But why? See post 21.

12-13-2019, 03:24 PM - 1 Like	#28
Wasp Pentaxian Join Date: Mar 2017 Location: Pretoria Photos: Gallery Posts: 4,659	Originally posted by bertwert But why? Because mathematics. This might help: https://damienfournier.co/dof-the-simplified-formula-to-understand-dof/

12-13-2019, 03:30 PM	#29
bertwert Veteran Member Join Date: Jun 2015 Location: Golden, BC Posts: 15,173	So if I had a K-1, and took a photo in APS-C crop mode, then took a photo in FF mode (from the exact same place with the exact same settings), then cropped the FF photo to be the same as the APS-C crop photo, the two photos would be different? I just can't wrap my head around it...

12-13-2019, 03:41 PM - 1 Like	#30
swanlefitte Pentaxian Join Date: May 2015 Location: Minneapolis Photos: Gallery Posts: 4,068	Originally posted by bertwert So if I had a K-1, and took a photo in APS-C crop mode, then took a photo in FF mode (from the exact same place with the exact same settings), then cropped the FF photo to be the same as the APS-C crop photo, the two photos would be different? I just can't wrap my head around it... Cropping the full photo and then enlarging it will change dof. Also printing larger or smaller than 8x10 will change Dof. And viewing from a closer or farther distance will change dof. Can you see when a point looks fuzzy or not is dof. Close up it's fuzzy at distance it's not. Large it's fuzzy and small it isn't. Dof is about perception of focus, not true focus.

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