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05-21-2014, 05:06 PM   #1
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Aperture and crop factor

Hello,
I watch this
and basically it shows that not only focal lenght has to be multiply by the crop factor, but also the aperture. It is possible to the get real aperture by

focal lenght (on 35mm) / lens diameter = aperture

I calculated it for some pentax lenses and just wonder whether it is correct, because most of the time I did not get the right aperture.

smc PENTAX DA Star 55mm F1.4 SDM
55mm (equiv. 82.5mm)
82.5mm/58mm=1.4224137931034482758620689655172 == 1.4

smc PENTAX DA Star 50-135mm F2.8 ED (IF) SDM
50-135mm (equiv. 75-203mm)
203mm/67mm=3.0298507462686567164179104477612 != 2.8

smc PENTAX DA Star 16-50mm F2.8 ED AL (IF) SDM
16-50mm (equiv. 24-75mm)
75mm/77mm = 0.97402597402597402597402597402597 != 2.8

smc PENTAX DA Star 60-250mm F4 ED (IF) SDM
60-250mm (equiv. 90-375mm)
375mm/67mm = 5.5970149253731343283582089552239 != 4

Thank you in advance.

Mic

05-21-2014, 05:13 PM - 1 Like   #2
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There is no crop factor or conversion for aperture. It's a constant and all cameras get the same amount of light.

The only thing is that you have more DOF at equivalent fields of view with smaller sensor sizes. At the same focal length, however, the DOF is the same since smaller sensors just see less of the frame.
05-21-2014, 06:47 PM   #3
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QuoteOriginally posted by Adam Quote
There is no crop factor or conversion for aperture. It's a constant and all cameras get the same amount of light.

The only thing is that you have more DOF at equivalent fields of view with smaller sensor sizes. At the same focal length, however, the DOF is the same since smaller sensors just see less of the frame.
What he said. Crop factor is just that, the frame dimensions expressed as a crop from the 35mm format. Focal length and relative aperture (f/stop) stay the same.


Steve
05-21-2014, 07:07 PM   #4
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QuoteOriginally posted by mictadlo Quote
I calculated it for some pentax lenses and just wonder whether it is correct, because most of the time I did not get the right aperture.
Where you went wrong is that you are using filter size (front element) as your diameter when you should be using the effective aperture diameter, not the lens or front element diameter. And the focal length marked on the lenses ARE for 35mm FF format, no need to apply crop factor.

---------- Post added 05-21-14 at 22:28 ----------

And I will add that the basic formula only works for prime lenses where the diaphragm does not move. The simple calculation will not hold true for most zooms (where the diaphragm may move) it is even worse when dealing with constant aperture zooms. I remember having the same question a few years ago and lens design, magnification, iris (aperture) position and nodal point are to be taken into account when calculating aperture size.

The focal length divided by aperture opening rule is an over simplification...


Last edited by fgaudet; 05-21-2014 at 07:29 PM.
05-21-2014, 07:38 PM   #5
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During his comparison of aperture between full frame and m43, he forgets that his focal lengths differ to render the same angle of view. That is why he has a deeper DOF with the m43. And ISO is actually an acronym for international organization of standards not "eyeso"....
05-21-2014, 07:39 PM   #6
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There is already a thread on this video:

https://www.pentaxforums.com/forums/169-pentax-full-frame/262253-pretty-good-...uivalence.html

if you can sift through the arguments you can find some good info.
05-21-2014, 07:50 PM   #7
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QuoteOriginally posted by manacho2005 Quote
And ISO is actually an acronym for international organization of standards not "eyeso"....
You might want to check that at the ISO website
About ISO - ISO

QuoteQuote:
Our name

Because 'International Organization for Standardization' would have different acronyms in different languages (IOS in English, OIN in French for Organisation internationale de normalisation), our founders decided to give it the short form ISO. ISO is derived from the Greek isos, meaning equal. Whatever the country, whatever the language, the short form of our name is always ISO.
05-21-2014, 09:55 PM - 1 Like   #8
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Warning rant to follow!

I really wish that the whole crop factor would simply go away. There is no such thing.

Lenses have two basic properties, focal length, and aperture which are completely independent of the format you work with. As pentax only has one format of DSLR, we waste a considerable amount of time 11 years after release of their first DSLR in discussing crop factor. Just forget you herd the dam expression and move on.

Aperture is simply focal length / diameter and when looking at lens dimensions, yes the front element can be used, but it only provides a limit it is not the true aperture when you take focal length over front element, but it does show the maximum real aperture that is possible. In the case of lenses beyond about 35mm it is a really good indicator, and for longer tele zooms it is also pretty reliable for the long end, but for many wide angle lenses the simple calculation is not always held due to different lens designs.

05-22-2014, 08:38 AM   #9
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QuoteOriginally posted by Lowell Goudge Quote
I really wish that the whole crop factor would simply go away. There is no such thing.

Lenses have two basic properties, focal length, and aperture which are completely independent of the format you work with. As pentax only has one format of DSLR, we waste a considerable amount of time 11 years after release of their first DSLR in discussing crop factor. Just forget you herd the dam expression and move on.
Hear! Hear!

Unfortunately, the term will continue to be with us as long as it is referenced in lens reviews and in the camera EXIF information and in the Wikipedia article on APS-C. It is so strange that almost 20 years after the definition of the APS-C format for film, we still reference FOV to the 35mm format. The term "crop factor" was originally coined by former 35mm shooters who could not figure out equivalent FOV when using the "new" system and only has relevance due to the two formats having the same 3:2 aspect ratio.

Since most users of Pentax dSLRs have never used or even held a 35mm SLR and have no frame of reference, it should be enough to simply classify a lens as ultra-wide, wide, normal, short/medium/long/extreme telephoto and qualify as applying to APS-C.


Steve
05-22-2014, 10:38 AM   #10
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QuoteOriginally posted by stevebrot Quote
we still reference FOV to the 35mm format
Mainly because it is still one of the de-facto format for many. It is odd that APS-C isn't the "main" format now, since there are a lot more APS-C cameras out there than FF. I wonder if the same thing will happen with people referencing FF focal length but for the 645 that everyone seems to be making now and that many user will adopt as the prices go down?

But I must admit that it is nice to have a point of reference. Especially when you're using a bunch of different formats. My main cameras are APS-C but I also use my Q often and my superprogram once in a blue moon (or every second blue moon nowadays). And then you add the GF micro four thirds to the mix and possibly a Q7 down the line, it gets messy. While having no reference, someone need to remember (or calculate) that if you want a "normal" lens, you could get a 50mm for the SLR, a 35mm for the APS-C, a 25mm for the m43, a 9mm on Q and a 11mm for the Q7... All giving the same field of view but a whole whack of different focal length. And what is wide, UWA, tele on each different format?

It's easy. simple math but if somehow there was a way to know, a reference maybe? Equivalence is here to stay. Using focal length "multiplier" as a reference is not the best since the actual focal length does not change... IMO using FOV in degrees would have been better. This is what actually matter, how much stuff will fit in your frame. 60 degrees FOV is 60 degrees FOV, no matter what format... but back in the days, it wasn't my call.
05-22-2014, 11:24 AM   #11
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QuoteOriginally posted by fgaudet Quote
But I must admit that it is nice to have a point of reference. Especially when you're using a bunch of different formats.
The elegant solution would be to use the angle of view. But that would mean a third parameter, linked to the body ANd lens, so it's not likely to happen. the crop factor is a constant conversion value so easier to compute.

Optics are so much more complicated than the two parameters (f-number, which we wrongfully call aperture, and focal length). Just to calculate DOF, the resolution of the sensor (or the pixel size, actually) plays a large role, which we constantly neglect.

But videos such as this one, mixing concepts to create an impact, are either misinformed or dishonest.
05-22-2014, 11:49 AM   #12
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QuoteOriginally posted by fgaudet Quote
Mainly because it is still one of the de-facto format for many. It is odd that APS-C isn't the "main" format now, since there are a lot more APS-C cameras out there than FF. I wonder if the same thing will happen with people referencing FF focal length but for the 645 that everyone seems to be making now and that many user will adopt as the prices go down?
No, what you will have is a complete screw up as people start referencing crop factor relative to 645 frame dimensions. If you are shooting large format you will reference to the film frame not to a different format all together. I can just hear it now.
05-22-2014, 01:14 PM   #13
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QuoteOriginally posted by fgaudet Quote
IMO using FOV in degrees would have been better
Yep. Though even that is a bit misleading depending on how you measure the angle and when comparing formats with different aspect ratio. I shoot 6x7cm and 4x5" formats in addition to APS-C and 35mm and have finally just resigned myself to learning my lenses/cameras. A good example would be the 4x5 view camera. In theory my 150mm lens is normal for that format, but in truth, it seems to shoot "wider" with that lens than what would be expected. The difference is in the aspect ratio.


Steve

---------- Post added 05-22-14 at 01:20 PM ----------

QuoteOriginally posted by Lowell Goudge Quote
I can just hear it now.
Equivalence ratios are sort of a fact of life in the large format world and most reference/learning books have a section on the subject. A good example would be my 4x5 field camera. It supports 4x5 sheet film, instant film holders in a variety of print sizes, and 120 roll film in a variety of frame sizes up to 6x12cm. Knowing what to expect from a given film holder is very useful at times


Steve
05-22-2014, 01:40 PM   #14
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QuoteOriginally posted by stevebrot Quote
It is so strange that almost 20 years after the definition of the APS-C format for film, we still reference FOV to the 35mm format.
As a reference format for FOV "equivalences,"
24x36mm has the advantage that it sits somewhere in the middle.

P&S formats to one side, view camera formats to the other.
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