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12-22-2007, 12:16 PM   #1
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Crop Factor: What's the deal?

Guys, what's the deal with crop factor? I just got my Pentax-M 50mm F1.7. I replicated shots that I had taken with my DA 18-55mm, and everything worked out fine. I then did a comparison by taking a shot with the DA 18-55mm at 50mm and the same shot with the M50. The shots were basically the same despite the difference in exposure. I really don't get what all the hoop-la about crop factor is. Somebody please enlighten me.

12-22-2007, 12:39 PM   #2
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Here is an explanation of crop factor.

The DA lens is designed with the crop factor in mind, the older ones are not. At least that is my understanding.

HTH

12-22-2007, 12:42 PM   #3
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I think you're confused about what a crop factor is. The first thing you need to know is that a 50mm lens is a 50mm lens is a 50mm lens regardless of what it's mounted on. The difference is the field of view.

If you mount the M 50/1.7 on a film body that takes 35mm film, the FOV will be roughly 'normal', IE neither wide nor telephoto. When you mount the same lens on your DSLR (which has a crop factor of ~1.5) the field of view (or the angle of biew if you prefer) will be narrower, you will have a short telephoto lens.

If you use film as a basis for comparison (as most still do) then to work out the effective FOV you multiply by 1.5. This is the part that people get confused about. What it means is that your 50mm lens WHEN MOUNTED ON YOUR K10D will have the same field of view as a 75mm lens mounted on a film body that takes 24x36mm (135 or 35mm) film.

A side issue that confuses people is image circle. New designed-for-digital lenses like the DA series do not generally have a large enough image circle to cover a traditional frame of 35mm film. If you put your 18-55 on a film body you will see severe vignetting, the corners will be completely black.

Matthew

EDIT: And of course someone's beaten me to it.
12-22-2007, 01:44 PM   #4
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So, would the FOV on my M50 be about the same as the FOV on a DA-70?

12-22-2007, 02:15 PM   #5
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QuoteOriginally posted by tux08902 Quote
So, would the FOV on my M50 be about the same as the FOV on a DA-70?
NO! it's gonna be different.
The crop factor is caused by smaller sensor/mirror box of DSLRs. The smaller size of these means that lenses on APS-C sized DSLRs have field of view of lenses with roughly 1.5x of their focal length on FF sised DSLRs of film SLR. So your M50 on any Pentax DSLR will have the FOV roughly equivalent to 75mm lens on FF sensor/film SLR...
12-22-2007, 02:27 PM   #6
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QuoteOriginally posted by tux08902 Quote
So, would the FOV on my M50 be about the same as the FOV on a DA-70?
The short answer is no.

I'm not sure the image circle is large enough on the DA-70 to cover a full frame 35mm film camera. But if it is, then the DA-70 mounted on a 35mm film camera would have approximately the same FOV as a M-50mm mounted on a Pentax DSLR. But the M-50mm on a DSLR does not equal the same FOV as a DA-70mm on a DSLR.

A 50mm lens is a 50mm lens, no matter what it is mounted on. Pentax DSLR cameras crop, film cameras do not. The cropping comes from the camera, not the lens.

The DA lenses are optimized for use on a DSLR. Which means they can make the lens smaller, because the image will be cropped on a DSLR, as compared to a full frame 35mm camera. The image circle produced by the lens does not have to be as large on a DSLR to cover the sensor.

HTH

12-22-2007, 02:39 PM   #7
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QuoteOriginally posted by tux08902 Quote
So, would the FOV on my M50 be about the same as the FOV on a DA-70?

No. Your M 50, on a digital camera with aps-c sensor, would have roughly the same FOV as a 77mm Full size lens mounted on your film camera. The DA lens are smaller and are optimized for the aps-c sensor.
12-22-2007, 02:53 PM   #8
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Oh, I see what this means. It's actually referring to the old standard for comparison. Does this really affect puchases? I mean I'm very happy with the M50. It does exactly what I bought it for.

12-22-2007, 03:06 PM   #9
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QuoteOriginally posted by tux08902 Quote
It's actually referring to the old standard for comparison. Does this really affect puchases? I mean I'm very happy with the M50. It does exactly what I bought it for.
Well, if you're happy, then I wouldn't about it too much!
12-22-2007, 03:09 PM   #10
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QuoteOriginally posted by tux08902 Quote
Oh, I see what this means. It's actually referring to the old standard for comparison. Does this really affect puchases? I mean I'm very happy with the M50. It does exactly what I bought it for.
It's not an old standard, as other aspects of photography are still true. Using the 1.5 x focal length method is only good for estimating FOV. DOF, perspective, etc, remains unchanged, 50mm is 50mm.
12-22-2007, 05:00 PM   #11
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QuoteOriginally posted by AVANT Quote
It's not an old standard, as other aspects of photography are still true. Using the 1.5 x focal length method is only good for estimating FOV. DOF, perspective, etc, remains unchanged, 50mm is 50mm.
the 1.5 x focal length also comes into play with respect to the recommended minimum shutter speed for hand holding.

Remember, you are really taking the center 66% of the image when compared to 35mm, therefore vibration effects are also amplified
12-23-2007, 09:08 AM   #12
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QuoteOriginally posted by Lowell Goudge Quote
the 1.5 x focal length also comes into play with respect to the recommended minimum shutter speed for hand holding.

Remember, you are really taking the center 66% of the image when compared to 35mm, therefore vibration effects are also amplified
This is true when the image is printed or viewed at the same size. If the M 50 image is increased in size to print 8x12, the image blur caused by shake would be multiplied by 8 on the 35mm camera, 12 on the APS-C sensor size in the dSLR.

I, personally, dislike the crop factor. It came in as a really well done sales pitch - my M 400 is now my M 600 - NOT. They decidedly played down the fact that my beautiful 24-90 became a 36-135, completely wiping out the wide angle side of things. So to get the same wide field of view, because of the smaller sensor size, I had to buy a 16-50, which is not really quite the same, as it has the fov of a 24-75, limiting the portrait end of things.
12-23-2007, 12:18 PM   #13
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So how does a Full frame sensor fit into all of this?
12-23-2007, 01:07 PM   #14
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Tamron has a link that gives a visual idea on the effect of the crop factor on angle of view.

Tamron - Focal Length Comparison
12-23-2007, 06:24 PM   #15
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So, which lenses do take into account the crop factor? Am I right in assuming that the DA/* aren't affected by the crop factor?
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