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05-06-2018, 03:56 AM   #1
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Crop Factor Conundrums

Hei, just a simple question about how the crop factor affects shooting, which I am probably overthinking:

I see lots of discussions about the f sweet spot for every lens in terms of CA's and sharpness. So, using an example:

Let say it is agreed that my 50mm f1.7 has the sweet spot between f5.6 - f8. Should I set the AR to f5.6 in order to achieve the desired performance, or is it enough to set it at, say f4 (f4*1,5=f6) so that the lens is behaving within its "sweet spot"?
I know the crop factor must be calculated to know the real aperture on APS-C sensors, but I am wondering if there is a difference between altering the physical shape of the aperture ring to a specific extent or simply accounting for the sensor crop (with a bigger shape on the aperture ring). As I said, probably overthinking, but you'll make me very happy if you can dispel this bothersome query.

Hope this makes sense,

Thank you for your input!

05-06-2018, 03:59 AM - 2 Likes   #2
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If your lens has a sweet spot between F5.6 and F8, then you set the aperture to be within that range. Sensor size/crop factor plays no role.
05-06-2018, 04:31 AM - 1 Like   #3
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The f-stop is a mathematical relationship between the focal length of a lens and the diameter of its opening. A setting of f4 means that the diameter of the lens opening is 1/4 of the focal length. Crop factor relates to the focal length of a lens in respect to the size of the sensor of the camera it's mounted on in comparison to a full-frame sensor. Crop factor and f-stop are not related in any way.
05-06-2018, 05:39 AM   #4
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Ah, understanding it correctly now, I see how there can be no confusion. Thanks!

05-06-2018, 05:54 AM - 1 Like   #5
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However... The diffraction effects are tied to pixel size which is related to sensor size. So a lens that has peak performance on the k-1 at f11 might peak at f8 on the KP since diffraction onset is earlier the smaller the pixel sites are.
05-06-2018, 06:13 AM - 3 Likes   #6
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Choice of aperture has three principle effects on a photograph: 1) exposure; 2) sharpness of in-focus areas; 3) relative bokeh size of out-of-focus areas.

Crop factor has no effect on exposure (you can use what a light meter tells you to use regardless of format size).

Crop factor has no effect on the sharpness of in-focus areas (use the lens' sweet spot regardless of format size).

Crop factor only affects the size of the bokeh blur relative to frame size. But even this is complicated because it is in the context of trying to create the identical image in two formats which requires using different lenses in those two formats. For example shooting a scene with an APS-C camera and 50 mm lens at f/4 would be replicated by shooting the identical scene with a FF camera and a 50*1.5 = 75mm lens set at 4*1.5 = f/6. These two shots would appear identical in terms of the amount of blur in out-of-focus regions in relation to the size of the final print.
05-06-2018, 06:39 AM   #7
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QuoteOriginally posted by photoptimist Quote
Choice of aperture has three principle effects on a photograph: 1) exposure; 2) sharpness of in-focus areas; 3) relative bokeh size of out-of-focus areas.

Crop factor has no effect on exposure (you can use what a light meter tells you to use regardless of format size).

Crop factor has no effect on the sharpness of in-focus areas (use the lens' sweet spot regardless of format size).

Crop factor only affects the size of the bokeh blur relative to frame size. But even this is complicated because it is in the context of trying to create the identical image in two formats which requires using different lenses in those two formats. For example shooting a scene with an APS-C camera and 50 mm lens at f/4 would be replicated by shooting the identical scene with a FF camera and a 50*1.5 = 75mm lens set at 4*1.5 = f/6. These two shots would appear identical in terms of the amount of blur in out-of-focus regions in relation to the size of the final print.
Thank you for a useful an educational reply Photoptimist. Comparatively this stands out as an example of how to appear knowledgable yet helpful and educated. Putting it together with UncleVanya's, I understand that I'll be better off by setting the aperture rings to whatever f stop is agreed as peak performance among crop sensor users. Then adjust exposure with all other variables
05-06-2018, 06:43 AM - 2 Likes   #8
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QuoteOriginally posted by Mørkus Quote
I know the crop factor must be calculated to know the real aperture on APS-C sensors, but I am wondering if there is a difference between altering the physical shape of the aperture ring to a specific extent or simply accounting for the sensor crop (with a bigger shape on the aperture ring). As I said, probably overthinking, but you'll make me very happy if you can dispel this bothersome query.
Unless you're mainly shooting with a FF camera do yourself a favor and just forget about "crop factor". It's meaningless if you're only shooting APS-C without never using a FF camera. Why would someone should care about the settings they would have used with a format they don't and have never used ?

With APS-C, the only things you need to know is that 35mm is the so called "normal" focal length, beyond 50mm is telephoto, 16-24mm is wide angle, below 16 is ultra wide. You don't have to care about what it would be in FF or any other formats.

Relation between aperture and sharpness has nothing to do with the crop factor since, anyway, this has to be determined on a individual lens basis because no two lenses are perfectly identical. So, do some tests with your own lenses and learn what are their sweet spots and the not so good ones. It's the only thing that matter: what are the real world performance of the gear you own. Not the supposed performance based on discussion in forums talking about other gear. Notwitstanding the fact that many differences turns out to be not significant in the real world. A lens sweet spot might be f/8, but might also be almost as good at f/2.8 to the point that it makes not practical differences. You then know that you can use this lens from f/2.8 with caring about some theoritical "sweet spot" based on lab tests...

Or, said otherwise, perfectly knowing your gear will be much more useful than anythig based on crop factor.

05-06-2018, 06:55 AM   #9
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QuoteOriginally posted by CarlJF Quote
Unless you're mainly shooting with a FF camera do yourself a favor and just forget about "crop factor". It's meaningless if you're only shooting APS-C without never using a FF camera. Why would someone should care about the settings they would have used with a format they don't and have never used ?

With APS-C, the only things you need to know is that 35mm is the so called "normal" focal length, beyond 50mm is telephoto, 16-24mm is wide angle, below 16 is ultra wide. You don't have to care about what it would be in FF or any other formats.

Relation between aperture and sharpness has nothing to do with the crop factor since, anyway, this has to be determined on a individual lens basis because no two lenses are perfectly identical. So, do some tests with your own lenses and learn what are their sweet spots and the not so good ones. It's the only thing that matter: what are the real world performance of the gear you own. Not the supposed performance based on discussion in forums talking about other gear. Notwitstanding the fact that many differences turns out to be not significant in the real world. A lens sweet spot might be f/8, but might also be almost as good at f/2.8 to the point that it makes not practical differences. You then know that you can use this lens from f/2.8 with caring about some theoritical "sweet spot" based on lab tests...

Or, said otherwise, perfectly knowing your gear will be much more useful than anythig based on crop factor.
I take the point on learning from experience and the individuality of every piece of gear. I am aware of this and use forum knowledge form experience.

Nevertheless, as I see it crop factor does become relevant when choosing what to invest my money in, since I will be limited in terms of maximum aperture and this will affect my range of choices to set the exposure. Light conditions are rarely optimal where I live and aperture makes a huge difference. Especially in buying old lenses designed for 35mm, I want to know what maximum effective aperture (or whatever you want to call this) I will be shooting with.
05-06-2018, 08:22 AM - 1 Like   #10
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Go here
Pentax K Lens Tests

For graphs like this


The first thing you learn from graphs like this is, it really doesn't make much difference. Shooting in the field you realize Depth of Field usually has more effect on the appearance of sharpness than does absolute sharpness. Personally, I'd use the above lens anywhere from ƒ4 to f-11, depending on how much depth of field I wanted for the shot.
I've done lots of images where I took 4 or 5 images ƒ2.8, 5.6 8, 11, 16. You rarely notice much difference in sharpness. You'll notice big differences in how the out of focus areas are rendered.

From my perspective, thinking about absolute sharpness, you're sort of barking up the wrong tree. The above chart can only be worthwhile photographing flat surfaces parallel to the sensor plane. On any other type of image how the scene is rendered is much more important. And that changes for each format at the same f-stop.

to be fair, my default settings on my camera are ƒ5.6 on APS_c and ƒ8 on the K-1, because that's where many of my images end up. Sharpest part of the lens in most cases (until you get really good lenses, like over $1000 dollars, although a DA 70 is sharpest at ƒ4.) And pretty much middle of the road DoF. So I start at the sharpest flat plane settings, then alter my aperture to achieve the rendering i want. With the above numbers being the defaults.

Last edited by normhead; 05-06-2018 at 08:31 AM.
05-06-2018, 08:27 AM   #11
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QuoteOriginally posted by Mørkus Quote
I take the point on learning from experience and the individuality of every piece of gear. I am aware of this and use forum knowledge form experience.

Nevertheless, as I see it crop factor does become relevant when choosing what to invest my money in, since I will be limited in terms of maximum aperture and this will affect my range of choices to set the exposure. Light conditions are rarely optimal where I live and aperture makes a huge difference. Especially in buying old lenses designed for 35mm, I want to know what maximum effective aperture (or whatever you want to call this) I will be shooting with.
But as said above by others, crop factor has nothing to do with exposure or the low light ability of a lens. Exposure values doesn't change between formats. Which is the goal of using the exposure system as we know it: a same set of exposure settings will give the exact same exposure no matter the format. It doesn't matter if the lens was designed for 35mm, modern digital, or any other formats or medium.

What is different with sensor formats are their sensitivities to low light. Which depends in part on format (bigger is usually better) and sensor technology (newer is usually better). But again, this has nothing to do with crop factor. For example, a K-P will give you better results in low light than many older FF camera...
05-06-2018, 09:12 AM   #12
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QuoteOriginally posted by CarlJF Quote
But as said above by others, crop factor has nothing to do with exposure or the low light ability of a lens. Exposure values doesn't change between formats. Which is the goal of using the exposure system as we know it: a same set of exposure settings will give the exact same exposure no matter the format. It doesn't matter if the lens was designed for 35mm, modern digital, or any other formats or medium.

What is different with sensor formats are their sensitivities to low light. Which depends in part on format (bigger is usually better) and sensor technology (newer is usually better). But again, this has nothing to do with crop factor. For example, a K-P will give you better results in low light than many older FF camera...
I believe I understand your input but I remain somewhat puzzled here. I have come across sources explaining that you have to multiply crop factor by aperture to understand your exposure. They say that even when the camera settings don't change, the smaller sensor means each pixel gathers less total light, even if the lens allows the same amount of light per square centimeter. The result is less total light in the image because there is less sensor size capable of gathering the light that comes in at a certain aperture. What am I missing?

To me this means that even if I own an f1.7 lens, I will never be shooting with the real world equivalent. Granted that if one has never experienced it, the equivalent of f1.7 with a crop may become their new "normal" definition of a 1.7, But in terms of learning and accuracy, that doesn't mean it represents what you can do on the same scene with f1.7 if you are really getting the exposure you would get with approximately f2.55 on FF.

EDIT: I just realised the crop factor multiplication is for bokeh and noise considerations, not exposure. Your answer was right on point and my big misconception prevented me from fully grasping it. Ignore what I said, my bad thanks for the patience!

---------- Post added 05-06-18 at 09:13 AM ----------

QuoteOriginally posted by normhead Quote
Go here
Pentax K Lens Tests

For graphs like this


The first thing you learn from graphs like this is, it really doesn't make much difference. Shooting in the field you realize Depth of Field usually has more effect on the appearance of sharpness than does absolute sharpness. Personally, I'd use the above lens anywhere from ƒ4 to f-11, depending on how much depth of field I wanted for the shot.
I've done lots of images where I took 4 or 5 images ƒ2.8, 5.6 8, 11, 16. You rarely notice much difference in sharpness. You'll notice big differences in how the out of focus areas are rendered.

From my perspective, thinking about absolute sharpness, you're sort of barking up the wrong tree. The above chart can only be worthwhile photographing flat surfaces parallel to the sensor plane. On any other type of image how the scene is rendered is much more important. And that changes for each format at the same f-stop.

to be fair, my default settings on my camera are ƒ5.6 on APS_c and ƒ8 on the K-1, because that's where many of my images end up. Sharpest part of the lens in most cases (until you get really good lenses, like over $1000 dollars, although a DA 70 is sharpest at ƒ4.) And pretty much middle of the road DoF. So I start at the sharpest flat plane settings, then alter my aperture to achieve the rendering i want. With the above numbers being the defaults.
This is very informational. I will have to take a closer look at this chart later on. Thank you very much!

Last edited by Mørkus; 05-06-2018 at 09:27 AM. Reason: Changed opinion / understood something new.
05-06-2018, 10:09 AM - 3 Likes   #13
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QuoteOriginally posted by Mørkus Quote
I believe I understand your input but I remain somewhat puzzled here. I have come across sources explaining that you have to multiply crop factor by aperture to understand your exposure.
These sources are wrong and go contrary to all photography textbooks. Exposure is defined by three variables: aperture, shutter speed and ISO. That's all. Sensor size (or film format) is not a variable and doesn't affect exposure.

QuoteOriginally posted by Mørkus Quote
They say that even when the camera settings don't change, the smaller sensor means each pixel gathers less total light, even if the lens allows the same amount of light per square centimeter. The result is less total light in the image because there is less sensor size capable of gathering the light that comes in at a certain aperture. What am I missing?
It's because they mix up noise and exposure. All other things being equal, for a same exposure, a larger sensor will give you less noise. But in the real world, all other things are never equal and sensor technology now plays a major part in how noise is handled, often trumping sensor size. For example, modern APS-C cameras now performed as good or better than older FF cameras of only a few years ago. Not even considering that different manufacturers will use different sensor technologies with different sensivities. And that a same manufacturer could use different sensor technologies for a same size sensor, often leading to quite different light gathering efficiencies. For all these reasons, crop factor consideration for exposure is at best totally irrelevant and at worst misleading.

QuoteOriginally posted by Mørkus Quote
To me this means that even if I own an f1.7 lens, I will never be shooting with the real world equivalent. Granted that if one has never experienced it, the equivalent of f1.7 with a crop may become their new "normal" definition of a 1.7, But in terms of learning and accuracy, that doesn't mean it represents what you can do on the same scene with f1.7 if you are really getting the exposure you would get with approximately f2.55 on FF
Again, with the same aperture, shutter spped abd ISO, this 1.7 lens will give you the exact same exposure on an APS-C than a FF camera. You will not get an underexposed picture on the APS-C camera. Exposure will not be more or less accurate on a APS-C than a FF, which depends on metering technology which has nothing to do with sensor size or crop factor. What the FF camera will give is the ability to push the ISO one stop higher than the APS-C before noise becomes unnacceptable, and assuming both cameras are using the same sensor technology.
05-06-2018, 10:59 AM - 2 Likes   #14
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The main thing to understand regarding crop factor is the field-of-view to expect through a lens of a given focal length. The rest sort of takes care of itself thereafter. As to the sharpness performance chart above, it can be helpful when choosing between lenses of the same FL for purchase. But as a photographic tool, be careful about limiting yourself to just using the sweet spot in practical terms. As was pointed out, the crop factor of that 50mm lens will provide an image comparable to that of a 75mm or 77mm lens on a FF body (I think the multiplier is x 1.53 for Pentax) when shot at the same distance, which means it can serve well for head and shoulder or even tighter portrait shots on an APS-C body. While the sweet spot is shown to be f/4-8, it would be fine to shoot such a portrait even wide open with this lens if wishing to blur the background to make the subject stand out more. This could apply also in other subject matter other than portraits. Note that even at f/1.8 central sharpness is still very good, and with a blurred background due to the wider aperture, the lower edge-of-frame sharpness will be unnoticeable. You can thus freely determine the degree of blur by the aperture chosen, f/1.8 here being the most extreme.

So do not limit yourself by your settings according to a certain lab standard. Rather consider your practical application.

Last edited by mikesbike; 05-06-2018 at 11:08 AM.
05-06-2018, 11:14 AM   #15
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QuoteOriginally posted by mikesbike Quote
The main thing to understand regarding crop factor is the field-of-view to expect through a lens of a given focal length. The rest sort of takes care of itself thereafter. As to the sharpness performance chart above, it can be helpful when choosing between lenses of the same FL for purchase. But as a photographic tool, be careful about limiting yourself to just using the sweet spot in practical terms. As was pointed out, the crop factor of that 50mm lens will provide an image comparable to that of a 75mm or 77mm lens on a FF body (I think the multiplier is x 1.53 for Pentax) when shot at the same distance, which means it can serve well for head and shoulder or even tighter portrait shots on an APS-C body. While the sweet spot is shown to be f/4-8, it would be fine to shoot such a portrait even wide open with this lens if wishing to blur the background to make the subject stand out more. This could apply also in other subject matter other than portraits. Note that even at f/1.8 central sharpness is still very good, and with a blurred background due to the wider aperture, the lower edge-of-frame sharpness will be unnoticeable. You can thus freely determine the degree of blur by the aperture chosen, f/1.8 here being the most extreme.

So do not limit yourself by your settings according to a certain lab standard. Rather consider your practical application.
Thank you for being helpful here, great to hear from experienced people

---------- Post added 05-06-18 at 11:16 AM ----------

QuoteOriginally posted by CarlJF Quote
These sources are wrong and go contrary to all photography textbooks. Exposure is defined by three variables: aperture, shutter speed and ISO. That's all. Sensor size (or film format) is not a variable and doesn't affect exposure.


It's because they mix up noise and exposure. All other things being equal, for a same exposure, a larger sensor will give you less noise. But in the real world, all other things are never equal and sensor technology now plays a major part in how noise is handled, often trumping sensor size. For example, modern APS-C cameras now performed as good or better than older FF cameras of only a few years ago. Not even considering that different manufacturers will use different sensor technologies with different sensivities. And that a same manufacturer could use different sensor technologies for a same size sensor, often leading to quite different light gathering efficiencies. For all these reasons, crop factor consideration for exposure is at best totally irrelevant and at worst misleading.


Again, with the same aperture, shutter spped abd ISO, this 1.7 lens will give you the exact same exposure on an APS-C than a FF camera. You will not get an underexposed picture on the APS-C camera. Exposure will not be more or less accurate on a APS-C than a FF, which depends on metering technology which has nothing to do with sensor size or crop factor. What the FF camera will give is the ability to push the ISO one stop higher than the APS-C before noise becomes unnacceptable, and assuming both cameras are using the same sensor technology.
Thanks. I did edit my reply stating that I basically understood your point right after posting.
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