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10-05-2010, 04:28 PM   #1
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A question for the experts

Many people use legacy lenses (with adaptors) on their DSLRs.

I understand that in most cases (except FF), ther is a "conversion factor" which crops the centre of the lens in such a way as to give the impression of a longer focal length (similar to having used a teleconvertor).

Alright, well here's a question that is not often asked: does this cropping also -simultaneously- result in an effective loss of light, i.e. in the apparent reduction of the lens's widest aperture (as it would with a TC)?

Thanking you for your response.

10-05-2010, 04:30 PM   #2
Ira
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QuoteOriginally posted by Banjo Quote
Many people use legacy lenses (with adaptors) on their DSLRs.

I understand that in most cases (except FF), ther is a "conversion factor" which crops the centre of the lens in such a way as to give the impression of a longer focal length (similar to having used a teleconvertor).

Alright, well here's a question that is not often asked: does this cropping also -simultaneously- result in an effective loss of light, i.e. in the apparent reduction of the lens's widest aperture (as it would with a TC)?

Thanking you for your response.
No.

A 1.4 is always a 1.4.

But it's a very good question and I understand why you asked it.
10-05-2010, 09:55 PM - 1 Like   #3
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Right down my alley! A 100/2.8 lens (for instance) remains a 100/2.8 lens no matter what camera body it's mounted on. Smaller-format frames (film or sensor) will see less of the image circle projected by the lens, but the lens' characteristics remain unchanged. The formatfaktor isn't a multiplier; it just indicates the cropping amount.

Try this: Cut out a picture from a magazine. Draw a 56x42mm rectangle on it; that's a 645 film camera's frame size. In its center, draw a 36x24mm rectangle; that's a FF (full-frame 135) camera's frame. In its center, draw a 24x16mm rectangle; that's about the size of an APS-C sensor like our dSLRs. THE PICTURE REMAINS THE SAME! The exposure (light and speed) and perspective haven't changed. But the smaller frames see less of it.

The formatfaktor is based on the diagonals of those frames, respectively about 70mm, 43mm, and 30mm. A formatfaktor tells you the ratio of the AoV (angle of view) in those format sizes. But that is all it tells you. The cigar is still just a cigar.
10-06-2010, 12:24 AM   #4
axl
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The above posters are right but:
To get exact replication of FOV with DOF you need faster lenses for APSC than for FF.
For example FF 77ltd shot at f1.8 would need APSC 50 shot at f1.2. If you would shoot the 50 at f1.8 you'd get dof equivalent to f2.4ish on 77 on FF...

10-06-2010, 02:38 AM   #5
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What people have not explained well is that when you use a teleconverter you project the light that is incoming over a larger area. That is why the image darkens once captured the enlargement from the sensor is not dependent upon available light at the scene. Cropping does not cut the light recorded at any point on the image but spreading the light that is incoming to the sensor over two e the area does
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