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Photography Question 
Dana  J. Lindberg

Digital vs. Normal SLR Lenses

I have an Olympus C8080wz. The aperture value of the lens is f2.4 - f8.0 with a 28mm - 140mm 35mm lens equivelancy. I have an old Pentax Spotmatic which came with a 50mm lens with aperture values of f1.4 - f16. Now the question! I was told that on a Digital SLR (i.e. Nikon D70) one could use 35mm AF lenses. But they would about "double" the focal length. Now if this is the case, could I consider the Olympus Lens to have a 35mm apeture value of approximately f1.2 - f16 with approximately a 56mm - 280 mm lens. I used to be able to make great photos without using the internal light meter and had much better control over depth of field, but digitally, things seem to have me a tad confused with the apertures and focal lengths. Thanks for the assistance.

Dana J. Lindberg

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3/16/2005 1:30:49 PM

Robert Hambley
  I don't know about the Nikons, but on the Digital Rebel I have, the smaller sensor (smaller than film) results in a 1.6 multiplication factor to the lens focal lenght. For example a 500mm lens becomes equivalent to an 800mm lens on a 35 film camera.

However, I don't believe the aperture values change.

So, if the Nikon had the same multiplication factor, the f2.4 -f8.0 28mm- 140mm would become a f2.4-f8.0
45mm - 224mm.

Hope this helps,

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3/16/2005 1:45:32 PM

Jon Close
BetterPhoto Member Since: 5/18/2000
  The focal length and other properties of a lens do not change whether it's used on a 35mm film v. digital camera. But the digital sensors are smaller than the 35mm film frame, so they record a smaller angle of view from the same focal length lens. They effectively "crop" a smaller image from the center of what the lens projects on the sensor/film plane. Since 35mm is so familiar to people, the lens focal lengths are converted to a "35mm equivalent" with the multiplication or crop factor, but this relates only to the angle of view. The actual focal length and aperture (and their relationships to depth of field and exposure) are unchanged.

The Olympus C8080 has a 2/3" digital sensor (2/3" is a measure carried over from vacuum tubes). Its image frame is just 8.8mm x 6.6mm, compared to 35mm film's 36mm x 24mm frame. The difference is equivalent to a crop factor of about 4x. The zoom lens of the C8080 has focal length of 7.13mm - 35.6mm. It gives equivalent angle of view as a 28-140 on a 35mm camera, but its aperture range is still f/2.4-8. f/2.4 is relatively "fast" with respect to exposure, but becaues of the lens's short focal length, it gives much greater depth of field than f/2.4-8 would with the equivalent 35mm film and lens. At the same focused distance, a 7mm lens at f/2.8 has the same depth of field as a 28mm lens at f/11. This is why you have more difficulty with getting shallow depth of field with the C8080 than you did with the Pentax Spotmatic.

Similar with the D70. Its APS-C sized digital sensor is smaller than 35mm film, just 23.7mm x 15.6mm. It uses the same lenses as the Nikon film cameras, but the angle of view captured by the smaller sensor is equivalent to a lens 1.5x longer on a 35mm camera. A 50mm f/1.4 used on it is still a 50mm f/1.4 with respect to exposure and depth of field. But on the D70 it gives the angle of view of 75mm on a N80.

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3/16/2005 6:50:19 PM

Robert Hambley

Jon got it alot more technically correct than I. Thanks for expanding and explaining where I missed.


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3/16/2005 7:59:15 PM

Dana  J. Lindberg   Thank you for you assistance. I didn't think that the focal length changed but I'm a lot less confused now. Thank you both for your responses.


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3/17/2005 7:32:35 AM

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