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09-17-2009, 12:55 AM   #1
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Focal length question

You probably get this question a lot so I'm sorry in advance for being a total newb, but I've tried Google but suprisingly couldnt find much info on the subject or at least nothing written in simple terms. I just want it to hear the answer right from the mouths of Pentax users I guess

My question is, when you are using a DA or DA* series lens, do you still have to factor in the crop factor for APS-C sensors?

I know when I'm using an FA lens I need to multiply by 1.5 to get the focal length that I'm actually using. But do I have to do the same multiplication to a DA or DA* lens?

So my 18-55 DA kit lens is really a 27-82.5, and my Tamron 90mm Di macro lens is really 135?

And if so, why do they designate it as FA and DA/DA*?

09-17-2009, 01:06 AM   #2
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The focal lengths of a lens remain the same on any camera but any lens on a Pentax DSLR has a cropped field of view by a factor of 1.5.

So yes, a 18-55 DA kit lens looks the same through a DSLR viewfinder as a 27-82.5 would on a film camera.

Last edited by Spock; 09-17-2009 at 03:48 AM.
09-17-2009, 01:26 AM   #3
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A bit more information:

The FA lens is designed to be used on a film camera or in the current parlance a "Full Frame" dSLR. A DA lens is designed for an APS-C dSLR.

A full frame sensor is essentially the same size as a 35mm film frame, whereas an APS-C sensor is smaller. Hence the name full frame.

When a lens projects the image onto the APS-C sensor the image that is captured by the sensor is smaller in size - in other words you get less of it than on a full frame. To explain if you were to cut a hole in a piece of card the same size as a 35mm film and place it over a picture of the same size you can see the whole picture inside the cut-out. If you then placed a card with an APS-C size hole cut in it over the same picture you would see a smaller amount of the picture as the hole is smaller. Then if you print both images to the same size paper, the APS-C image will look like it was taken with a longer zoom lens as the components of the image will look larger (closer) than that of a full frame. Hence the crop factor - the image appears cropped by a factor of 1.5 on an APS-C sensor.

Now back to the difference between FA and DA - a DA lens will project an APS-C size image onto the sensor, whereas an FA will project a larger image but the sensor will only capture the amount that the sensor sees.

So.... if you were to place a DA lens on a Full Frame camera (or a 35mm) there would be a black frame around the captured image - the lens only projects an APS-C size image.

Hope this helps...
09-17-2009, 01:53 AM   #4
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Thanks for the info, that clears it up. So given that a FA lens covers both fields of view, and the DA lens will only cover an APS-C sensor, is there any point to collect DA lenses (other than for a specific need like the weather-proofing on DA*'s), in the context of Pentax some day releasing a full frame dslr and wanting to have my collection be compatible (I know there are nothing but rumors and no guarantee that Pentax will ever release one).

Also, I take it then that no matter which brand of camera you buy (Pentax, Canon, Nikon, etc) no matter what the designation or type of lens, you always have to do the crop factor conversion yourself, and the actual APS-C focal length is never listed on a lens?

09-17-2009, 03:35 AM   #5
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Sorry to disappoint you but pentax marketing (or what ever they call it these days) has been absolutely clear on full frame for years.

'they are comitted to the ASP-C sensor for the foreseeable future with no full frame camera in development,"

so first of all, I would not be comitting to full frame only lenses if all you are going to use is a DSLR.

Even if pentax does come out some day with full frame, they will make the lenses useable with some restrictions, just like nikon and canon do, and some of the DA lenses will actually cover fulll frame, and there is a forum thread that explores this.

BUT, i think sensor technology will continue to evolve and full frame will become an obsolete format, where medium format will take over as these cameras develop

In my lens collection, I have only one ASP-C lens, a sigma 10-20mm, BUT that is my choise, not because I am waiting for a full frame camera, but because I also shoot film from time to time, and use a PZ-1 for AF or Ricoh XR2s for MF lenses. Therefore for me, full frame lenses are so I can shoot film, not because I am waiting for a full frame DSLR
09-17-2009, 12:49 PM   #6
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QuoteOriginally posted by McLovin Quote
Thanks for the info, that clears it up. So given that a FA lens covers both fields of view, and the DA lens will only cover an APS-C sensor, is there any point to collect DA lenses (other than for a specific need like the weather-proofing on DA*'s), in the context of Pentax some day releasing a full frame dslr and wanting to have my collection be compatible (I know there are nothing but rumors and no guarantee that Pentax will ever release one).
If you're dying to buy a FF camera for whatever reason should one become available, then by all means, avoid DA lenses. Although you should probably also avoid Pentax, since it's pretty unlikely they'll release an FF camera. However, even if they do release an FF camera, I'm pretty sure they can't force anyone to buy it if you're happy with APS-C.

QuoteQuote:
Also, I take it then that no matter which brand of camera you buy (Pentax, Canon, Nikon, etc) no matter what the designation or type of lens, you always have to do the crop factor conversion yourself, and the actual APS-C focal length is never listed on a lens?
The actual focal length *is* what is listed on the lens. Focal length is a physical property of the lens and crop factors don't affect it at all. A 50mm is always a 50mm lens, regardless of what camera you mount it on. Crop factors are just a way of comparing the field of view provided by the lens on different sized formats. So no, camera companies do not post such conversion tables on their lenses. No more than the US Treasury prints conversion tables for their dollar bills telling you how much it would be worth in Euros, or breakfast cereals are marked with what the weight of the box would be on the moon.
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