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03-23-2008, 03:28 PM   #1
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yikes how long is my lens ?

I have a K10D and a Tamaron 70-300, but is that 70-300 in consideration of the 1.5 crop factor of the K10D sensor ?

I'm sure I have taken two pictures of the same distant building from the same point with my K10D and my old fuji S6500fd also at 300 mm. the K10D + a tamaron 70-300 at 300 gives a closer up shot than the fuji.

so what about my pentax kit 18-55 lens, what the hec is going on ? what lenses do I really have ?

03-23-2008, 03:35 PM   #2
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kit lens = 27 - 82.5mm
Tamron = 105 - 450mm

Just multiply all focal lengths on the lens by 1.5 to get the effective focal length on the camera.
03-23-2008, 03:38 PM   #3
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ah great how nice of them to forget to mention trhis little detail, so what about apertures ? dosn't that mean that the aperture is in efect half ?
03-23-2008, 03:51 PM   #4
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well its nice to know i now have a 450 mm lens but cmon how many people still use film isn't it time to move on, as far as I know all pentax DSLRs are 1.5 crop, or put two blasted scales on the lens just to make life simpler, so its a case of shutter speed = 2 X focal length, that explains the slight blur in some of my pictures

03-23-2008, 03:55 PM   #5
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A 70-300mm lens will always be a 70-300mm lens - nothing more and nothing less.

It does not magically transform into a 105-400mm lens when you slap it on your Pentax DSLR.

The Pentax with a 1.5X crop factor will give the appearance that a longer focal length lens is being used, but it is basically the same picture cropped. There is tons of information on crop factors on the net, below are a couple examples.

Crop factor - Wikipedia, the free encyclopedia

The digital crop factor explained - Fred Kamphues
03-23-2008, 03:55 PM   #6
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Aperture ranges remain as stipulated.
03-23-2008, 04:04 PM   #7
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yea i understand the crop factor thing and i tend to reason in terms of 50 mm as the "normal" but now I have to start thinking in terms of 33 mm as being the normal.

if the image is croped by 1.5 (and 1.5 X 1.5 = 2.25) then its receiving proximately half the light isn't it ? as the light outside the sensor area wich is half the area of a film frame does not come into the image anymore. erm maybe I'm talking jiberish actually = bed time after I've read those links
03-23-2008, 04:11 PM   #8
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so there are some (exspensive) cameras that have a full frame sensor (yea bashes sleepy head on wall - of course there are also sensors that are 35 mm) so we still have lenses made for 35 mm format and "have" to be reminded that we are not rich enough to get such cameras and have to be contented with cheaper APC sensor cameras. as far as I know pentax does not make a full frame sensor camera ?

03-23-2008, 04:38 PM   #9
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QuoteOriginally posted by simons-photography Quote
yea i understand the crop factor thing and i tend to reason in terms of 50 mm as the "normal" but now I have to start thinking in terms of 33 mm as being the normal.

if the image is croped by 1.5 (and 1.5 X 1.5 = 2.25) then its receiving proximately half the light isn't it ? as the light outside the sensor area wich is half the area of a film frame does not come into the image anymore. erm maybe I'm talking jiberish actually = bed time after I've read those links
While the image is receiving half the "light" in total, with the croppped sensor, the intensity of light per square mm does not change, that is why your exposure does not change.

You need to simply consider focal length is always focal length, it is the distance (for a simple lens) that the lens is away from the plane of focus, for an object at infinite distance. Because this is a physical property of the lens, it cant change, it is independant of the sensor size.

F Stop similairly is a fixed parameter of a lens. Again consider a simple lens, it is the diameter of the lens divided by the focal length. Again this cant change and is independant of the sensor size. The F-Stop of a lens really is only an indication of how bright an image is, and this brightness is based upon the intensity of light per square mm.

With respect to the crop factor and focal length, it is better to forget all about "equivelent focal length" what really changes is the field of view, that is all. an ASP C sensor crops out the middle of a 35 mm frame, hence with the same lens it is clear that the angle of view on an ASP-C sensor is less than a 35mm or full frame sensor. In terms of printing, to print up to a specific size image, the smaller sensor requires more magnification in processing.
03-23-2008, 09:25 PM   #10
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QuoteOriginally posted by simons-photography Quote
yea i understand the crop factor thing and i tend to reason in terms of 50 mm as the "normal" but now I have to start thinking in terms of 33 mm as being the normal.

if the image is croped by 1.5 (and 1.5 X 1.5 = 2.25) then its receiving proximately half the light isn't it ? as the light outside the sensor area wich is half the area of a film frame does not come into the image anymore. erm maybe I'm talking jiberish actually = bed time after I've read those links
Take note that a 4 inch lens (100 mm) is a normal lens on a 4x5. The focal length affects the field of view, agreed, but the fov is also affected by the size of the receiving medium. So people throw around the "Your 200 mm lens is now a 300 mm lens" when it is not. The statement should read "Your 200 mm lens, when used on an APS-C sensor digital camera, has the field of view that a 300 mm lens has on a 35mm camera's 24x36 mm film gate."

The camera makers kept emphasizing how great the crop factor was, and conveniently ignored the consequences at the wide end of things, where your 24 mm lens now was cropped down to the middle two thirds of the image, and now you need a 16mm lens to get the same scene into the image.
03-23-2008, 09:50 PM   #11
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QuoteOriginally posted by Canada_Rockies Quote
Take note that a 4 inch lens (100 mm) is a normal lens on a 4x5. The focal length affects the field of view, agreed, but the fov is also affected by the size of the receiving medium. So people throw around the "Your 200 mm lens is now a 300 mm lens" when it is not. The statement should read "Your 200 mm lens, when used on an APS-C sensor digital camera, has the field of view that a 300 mm lens has on a 35mm camera's 24x36 mm film gate."

The camera makers kept emphasizing how great the crop factor was, and conveniently ignored the consequences at the wide end of things, where your 24 mm lens now was cropped down to the middle two thirds of the image, and now you need a 16mm lens to get the same scene into the image.
except that your perspective does not change with the change in field of view, since it's just cropping down the image, therefore it's only a 1.5x FOV modifier..
03-23-2008, 09:54 PM   #12
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QuoteOriginally posted by OniFactor Quote
except that your perspective does not change with the change in field of view, since it's just cropping down the image, therefore it's only a 1.5x FOV modifier..
Agreed. Left that out. My bad.
03-23-2008, 10:04 PM   #13
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hehe. no problem. it's just that i hate when people refer to it as a focal length modifier, when it's just a CROP. if you were to put a 300mm lens on a film body, take a picture, take a 200mm mm lens on a APS-C (1.5x) body, take the same picture, it will NOT look the same, because of this little thing called background seperation. i wish i had a film pentax body to show this effect to people..
03-23-2008, 10:19 PM   #14
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QuoteOriginally posted by OniFactor Quote
hehe. no problem. it's just that i hate when people refer to it as a focal length modifier, when it's just a CROP. if you were to put a 300mm lens on a film body, take a picture, take a 200mm mm lens on a APS-C (1.5x) body, take the same picture, it will NOT look the same, because of this little thing called background seperation. i wish i had a film pentax body to show this effect to people..
You can do it with the digital. Just take two shots, and crop one to the same fov as the other. For example, take a shot at 100 mm, crop it to 200 mm fov. then take a shot with the 200 from the same place. You get the same perspective. It's the opposite of what you wrote, but does show the same thing.
03-24-2008, 12:41 AM   #15
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ok thanks for all you help guys yes the f stop should be unchanged, I think I'll consider some secondhand film lenses at this point. any tips there ?
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