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01-22-2013, 06:27 PM   #1
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Absolute or relative aperture for adapted lenses?

Hi, here's a small observation I did before going to bed last night, and I think it does seem to make sense. I had too much caffeine earlier on

I think when adapting lenses to the Q, one must take into account that the native Q lenses are in fact very tiny. The 8.5mm standard prime has a maximum aperture of 1.9. I compared it to a K-mount lens with an aperture of 2.0, it's obvious that the aperture of the SLR lens is much bigger. To create an aperture size on the K-mount lens to match the maximum Q lens aperture fully open, I had to stop down the SLR lens to appoximately 5.6.

So it struck me that some members have reported poor image contrast, excessive fringing and abberation when using the adapted K mount lens may be due to the aperture being too large wide open. Since 5.6 on a K is equivalent to 1.9 on a Q, then 2.0 on a K is maybe 0.6 on a Q?

From my observations, it appears that the effective aperture of a lens is also relative to the sensor size. To the Q users who already have the K-Q adapter, it may be worth to try stopping down from 8 all the way to 32 and see if you get any better results. When you do, please post some photos

01-22-2013, 06:34 PM   #2
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QuoteOriginally posted by weijen Quote
Hi, here's a small observation I did before going to bed last night, and I think it does seem to make sense. I had too much caffeine earlier on

I think when adapting lenses to the Q, one must take into account that the native Q lenses are in fact very tiny. The 8.5mm standard prime has a maximum aperture of 1.9. I compared it to a K-mount lens with an aperture of 2.0, it's obvious that the aperture of the SLR lens is much bigger. To create an aperture size on the K-mount lens to match the maximum Q lens aperture fully open, I had to stop down the SLR lens to appoximately 5.6.

So it struck me that some members have reported poor image contrast, excessive fringing and abberation when using the adapted K mount lens may be due to the aperture being too large wide open. Since 5.6 on a K is equivalent to 1.9 on a Q, then 2.0 on a K is maybe 0.6 on a Q?

From my observations, it appears that the effective aperture of a lens is also relative to the sensor size. To the Q users who already have the K-Q adapter, it may be worth to try stopping down from 8 all the way to 32 and see if you get any better results. When you do, please post some photos
Aperture is not related to the sensor size. It is a ratio based on the area of the front element and focal length of the lens. Think of it this way: a really wide lens can capture a lot of light from all different directions, and thus doesn't need as much glass. On the other hand, longer lenses need very large front elements in order to harvest just as much light from a smaller effective field of view. Even if the "hole" in the diaphragm of a K-mount lens is bigger than that of a Q lens, the aperture value may be much smaller.

Since Q lenses only go to F8 but K-mount lenses will go lower, you will be able to observe much more diffraction if you stop down all the way.

I believe that excess fringing is often visible because of the fact that older lenses have worse coatings than modern lenses, and the aberrations are getting blown up because you're packing 12 megapixels into such a small portion of the frame.

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01-22-2013, 07:50 PM   #3
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Aperture is aperture. Focal length is focal length. F-number is the ratio of focal length to aperture diameter (hence the convention of expressing it as a fraction). Neither is related to sensor size.


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01-22-2013, 09:29 PM   #4
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QuoteOriginally posted by weijen Quote
From my observations, it appears that the effective aperture of a lens is also relative to the sensor size. To the Q users who already have the K-Q adapter, it may be worth to try stopping down from 8 all the way to 32 and see if you get any better results. When you do, please post some photos
As others have mentioned, aperture is aperture.

However the diffraction limit matters more on tiny pixels like the Q. If you stop down all the way to F/32 you'll be tremendously soft on the Q just due to diffraction alone, even with perfect optics.

01-23-2013, 01:09 AM   #5
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Ok, thanks for the input. I stand corrected. It was just a thought while waiting for my adapter to arrive

So, a final question just to wrap things up: When the K mount lens is set to f8, would the Q also receive as much light as f8 on it's native Q lens?
01-23-2013, 06:20 AM   #6
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QuoteOriginally posted by weijen Quote
Ok, thanks for the input. I stand corrected. It was just a thought while waiting for my adapter to arrive

So, a final question just to wrap things up: When the K mount lens is set to f8, would the Q also receive as much light as f8 on it's native Q lens?
Yes the amount of light reaching the sensor is the same for a K mount lens @ f8 and a Q lens @ f8

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01-24-2013, 08:02 AM   #7
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Aperture is, as said, usually expressed as f/x which gives the opening size, and this is more useful than the actual size for various reasons having to do with physics.

But this question gives me the perfect excuse to rant a bit, so I will.

Lots and lots of people like scaling the focal length by sensor size, and it always seems stupid to me not to scale the f number as well. If I want to claim that the 01 standard prime is like a 47mm lens, I should also say it's like an f/10.5 lens, because it is. (The opening didn't get bigger just because you decided to play with a number.) And then some, uh, very fine gentleman will invariably point out that this messes up exposure calculations, because scaling the ISO as well is of course completely and utterly out of the question.

But that's wrong, scaling the ISO is just as valid. The Q with 01 standard prime is a camera with base ISO 125 and an 8.5/1.9 lens. This is equivalent to an FF camera with base ISO 4000 and a 47/11 lens (rounded). And this comparison is perfectly reasonably to make if you want to understand what it's capable of and you're used to thinking about cameras with FF size sensors. (But it is not some special truth about the camera that is more valid than scaling it for example to being like and APS-C camera with a 31/7 lens, just an arbitrary standard some people agree on using as a reference.)
01-24-2013, 02:32 PM   #8
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QuoteOriginally posted by drougge Quote
But that's wrong, scaling the ISO is just as valid. The Q with 01 standard prime is a camera with base ISO 125 and an 8.5/1.9 lens. This is equivalent to an FF camera with base ISO 4000 and a 47/11 lens (rounded).
I understand the scaling of the focal length to adjust for sensor size. I also understand the aperture scaling to account for depth of field. I don't understand scaling the ISO. Maybe my mind is stuck on how ISO relates to exposure, I don't know. I know aperture scaling has nothing to do with exposure and I get that, but why would ISO need scaling too?

01-24-2013, 05:39 PM   #9
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QuoteOriginally posted by post_eos Quote
I understand the scaling of the focal length to adjust for sensor size. I also understand the aperture scaling to account for depth of field. I don't understand scaling the ISO. Maybe my mind is stuck on how ISO relates to exposure, I don't know. I know aperture scaling has nothing to do with exposure and I get that, but why would ISO need scaling too?
But scaling the aperture does have everything to do with exposure, and somewhere around here is where people tend to get confused. If you scale any of the numbers, you should scale all three. Let me give examples:

You shoot your 8.5mm lens at f/2.8 at ISO 125 on your Q. The shutter speed for good exposure is 1/250.

Then you try to take an equivalent picture on your K-5. You use your 31mm lens at f/11. If you still want 1/250 shutter speed you need ISO 2000.

If you instead of actually taking these images talk about your Q as if it was an APS-C camera, you should still scale all three numbers, because anything else will give the wrong idea of what the picture will look like. If you say it's a 31mm lens shot at f/2.8 for 1/250s you get the right exposure at ISO 125, but the DOF is wrong. So you scale the f-stop and say you shoot it at f/11 instead, but to get the right exposure you now need ISO 2000.

And all of this is for when explaining what it's like for someone used to another format. Scaling it to some format none of the talking parties ever use is just silly.

Last edited by drougge; 01-24-2013 at 06:44 PM. Reason: grammar
01-24-2013, 05:49 PM   #10
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QuoteOriginally posted by drougge Quote
Let me give examples:

You shoot your 8.5mm lens at f/2.8 at ISO 125 on your Q. The shutter speed for good exposure is 1/250.

Then you try to take an equivalent picture on your K-5. You use your 31mm lens at f/11. If you still want 1/250 shutter speed you need ISO 2000.
Got it. That makes sense now. Thanks for the explanation.
01-25-2013, 05:23 AM   #11
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Some confusion here, aperture size does vary with focal length, but also with the size of the projected image the lens creates, smaller optics produce a smaller image circle and need smaller apertures.

It's the area of aperture that matters, but the 35mm format happens to directly correlate pretty closely aperture diameter to focal length (which is one reason why it's used as the base line), a 35mm format, 50mm lens at f8 will have a 50/8=6.25mm diameter aperture. It will be f8 lumens per square mm no matter what camera you attach it to. But smaller optics changes things.

Lenses designed for other sensor formats you have to divide by the crop factor (from the standard 35mm format) too. Sometimes this is taken care of if you alter the focal length to take account of the sensor size, for instance the Pentax Q 8.5mm (similar angle of view as a 50mm lens in 35mm format) at f8 would be 8.5/8=1.062mm diameter aperture, which is why smaller apertures than that have diffraction problems on a Q.

If you do the maths on a 47mm (in 35mm format) which is the direct equivalent of the Q 8,5mm lens and divide by the crop factor you get 47/8/5.5= you get 1.068mm, the slight difference is because I'm using the rule of thumb of the 35mm format of dividing the focal length by the aperture = the diameter of the aperture, not perfect, but, as I said, pretty close.

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01-26-2013, 12:42 PM   #12
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QuoteOriginally posted by ChrisJ Quote
Some confusion here, aperture size does vary with focal length, but also with the size of the projected image the lens creates, smaller optics produce a smaller image circle and need smaller apertures.
There is certainly some confusion, but I think it's mostly yours. The size of the physical aperture for the same f-number depends on focal length, certainly, but this has nothing to do with the image circle, or anything I talked about.

QuoteOriginally posted by ChrisJ Quote
It's the area of aperture that matters, but the 35mm format happens to directly correlate pretty closely aperture diameter to focal length (which is one reason why it's used as the base line), a 35mm format, 50mm lens at f8 will have a 50/8=6.25mm diameter aperture. It will be f8 lumens per square mm no matter what camera you attach it to. But smaller optics changes things.
A 50mm f/8 lens will have a 6.25mm aperture regardless of what format the lens is for.

I have no idea what you're going for here. Your numbers are true, except I don't know what "f8 lumens" is supposed to mean. It will certainly give the same amount of light per area regardless of format (as long as you stay in the good part of the image circle).

QuoteOriginally posted by ChrisJ Quote
Lenses designed for other sensor formats you have to divide by the crop factor (from the standard 35mm format) too. Sometimes this is taken care of if you alter the focal length to take account of the sensor size, for instance the Pentax Q 8.5mm (similar angle of view as a 50mm lens in 35mm format) at f8 would be 8.5/8=1.062mm diameter aperture, which is why smaller apertures than that have diffraction problems on a Q.
The size of the aperture (let's ignore the virtual aperture on complex lenses, as we have so far) is not actually the determining factor for diffraction, or longer lenses would have less diffraction for a given f-number, which is not true. (It is sometimes a little true, but nowhere near what the aperture size would yield.) I admit the physics here are a bit fuzzy to me, but it works for me to think of it like this:

The aperture on a longer lens is larger, but also further away, giving the light rays more spread for the same amount of trajectory change. (It would be a perfectly good rule on simple lenses, why it still seems to apply so well to complex lenses is a bit unclear to me.)

QuoteOriginally posted by ChrisJ Quote
If you do the maths on a 47mm (in 35mm format) which is the direct equivalent of the Q 8,5mm lens and divide by the crop factor you get 47/8/5.5= you get 1.068mm, the slight difference is because I'm using the rule of thumb of the 35mm format of dividing the focal length by the aperture = the diameter of the aperture, not perfect, but, as I said, pretty close.
Your "rule of thumb" is true for simple lenses, and we are treating the lenses as simple. Rounding errors account for the difference.
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