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09-07-2012, 04:30 AM   #1
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100mm F2.8 Macro image size?

Hello Folks,

According to the lens review page here, the image circle of the Pentax-D FA 100mm F2.8 Macro WR covers the full 35mm film format. My understanding is that the CCD chip in my K-5 is smaller than 35 mm. Does this mean that the image will be cropped in the camera?

Thanks, Tom

09-07-2012, 04:35 AM   #2
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yes - FOV equivalent to 150mm (16 degrees instead of 24degrees diagonal)
09-07-2012, 05:06 AM   #3
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Yes, but your 100mm lens is still a 100mm lens! Meaning if you take a lens for digital at 100mm you would get the same field of view (half the one of any 50mm lens).
If you only have a digital SLR don't worry about this kind of stuff.
However, if you have or had a film SLR, then consider that all 100mm lenses will bring equivalent results to what you used to have with a 150mm lens.
Bottom line: the relationship between focal length and field of view depends on the camera, but NOT on the lens age, series or design!
09-07-2012, 05:11 AM   #4
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So, does that mean that the Pentax-D FA 100mm F2.8 Macro WR will have an effective focal length of 150 mm when mounted on my Pentax K-5?

09-07-2012, 05:19 AM   #5
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Well in a way yes, just like:
- your 18-55mm zoom has an effective zoom range of ~28-80mm (35mm eq.)
- any 35mm has an effective focal length of ~50mm (35mm eq.)
- any 50mm has an effective focal length of ~75mm (35mm eq.)
when mounted on a Pentax DSLR.
09-07-2012, 05:56 AM   #6
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Simply put, since the image sensor is smaller it percieves less of the light delivered by the FF lens. This would be like using a 150mm lens. However, you do NOT get extra magnification just by mounting a FF lens on a crop sensor - it's still 100mm. The 100mm macros are still great lenses for insects and other items which need a little "stand-off" distance.
09-07-2012, 06:22 AM   #7
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God, but I hate effective focal length discussions.

The simple fact is, that image size is not relevant to the image circle a lens produces.

On your K5, your viewfinder is a 100% viewfinder, which means that what you see in the viewfinder is exactly the image projected onto the sensor.

the Pentax-D FA 100mm F2.8 Macro WR lens is a 100mm macro which is cabable of 1:1 reproduction, i.e. can produce a life size image at minimum working distance

that is all that matters.

forget about image circle unless you intend to use this lens on a film body, or ulness pentax at some time in the future releases a full frame DSLR. but regardless of what camera you use the lens on, that changes nothing. what you see in the viewfinder is what you get, nothing more, nothing less.
09-07-2012, 07:10 AM   #8
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No, your image won't be cropped, you'll still get a full image. But, it could also cover a 35mm image fully if it were on a full frame camera. Full frame has a larger sensor area than k5 so actually you see more hence k5 is said to be cropped.

Forget about the 100/150mm thing, that's just marketing rubbish. Its a 100mm lens, but when used on apsc it has a smaller field of view than when used on 36x24. So, marketers being marketers turn that around and call it 150mm (because that also has a smaller field of view).

But fyi, a full frame lens on a k5 is actually a really good thing, because rather than using the edges of the glass which tends to be worse, you're using the centre, it's sweetspot.


Last edited by Nass; 09-07-2012 at 07:15 AM.
09-07-2012, 07:44 AM   #9
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QuoteOriginally posted by Nass Quote
No, your image won't be cropped, you'll still get a full image. But, it could also cover a 35mm image fully if it were on a full frame camera. Full frame has a larger sensor area than k5 so actually you see more hence k5 is said to be cropped.

Forget about the 100/150mm thing, that's just marketing rubbish. Its a 100mm lens, but when used on apsc it has a smaller field of view than when used on 36x24. So, marketers being marketers turn that around and call it 150mm (because that also has a smaller field of view).

But fyi, a full frame lens on a k5 is actually a really good thing, because rather than using the edges of the glass which tends to be worse, you're using the centre, it's sweetspot.
while this is true, it is also true that a full frame lens is designed to project a full image circle, therefore there is a lot of stray light coming out of the rear of the lens that will be bouncing off every surface in the mirror box, reducing contrast, but also the internal baffling is designed for a wider field of view, this means there is light entering the lens that does not contribute to the image, but which could result in internal flare and lost contrast. although coatings and internal baffling are pretty good, use of a lens hood that is restricted to the FOV on digital may result in increased contrast
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