[Doctor-CTAC]

1. When zooming in, why must the minimum apeture get smaller and smaller as the camera zooms?
2. I have a Pentax K100D and I'd like to get a nice portrait lens. What would you recommend? How can I be sure the lens will fit on the camera, do I just go by the lens size (58mm I think on the k100D) or is there something else I should look out for?

1. That's simply due to focal length is basically the diameter of the lens multiplied by the F stop. So basically, there are two ways to increase focal length, either increase the F/stop, or increase the diameter. However, increasing the diameter of the lens would be costly, as glass is what makes up most of the cost of the lens. Therefore, in cheaper lens, manufacturers simply just increase the F/stop.

2. I find the 50mm F/1.4 very nice for portraits on the APS crop. It gives milky smooth bokeh, and very fine colors.
If you want more of a blurrier background, you could get a longer lens, such as an 105mm F/2.8, or a 135mm. You can find these focal lengths in pentax's plethora of manual focus lenses for bargain prices.
If you have more to spend, you could go for the 85mm, but even the old m42 version of this lens goes for $300 on ebay! But it comes with a legendary reputation .
And if you really want the ne plus ultra, you could either get the 85mm F/1.4 star lens, or the 77mm F/1.8 limited. But either of these would run you at least $700.

[nvideoe]

So I went to the local camera shop and the guy there said he preferred 75-100mm lenses for portrait photography. I didn't really understand, why would you want a 75 or 100mm lens X 1.5 crop = 150 - 200mm lens for taking pictures? I asked him about a small lens like a 28mm and he said I wouldn't want 28mm because there would be too much distortion. Most of the posts here have said 50mm is the way to go, but why shouldn't I get a 28mm or less?

I just want to point out (since it's a pet peeve of mine) that crop factor does NOT change the focal length of a lens!!! A 50mm lens on a 1.6x crop camera is still taking photos at 50mm. It simply cuts the edges of the photo out. There's this weird misconception out there that cropping is identical to having a lens with a higher focal length, and it's simply not true.

If you dont believe me, put a zoom lens on your 1.6x crop body and zoom to 30mm. According to some people, this should make the lens an effective 50mm. But if you look through the viewfinder, you'll see that the field of view is zoomed out and objects appear smaller than they do with your naked eye (and 50mm is supposed to give the same field of view as your naked eye). Now change your zoom to 50mm, and you'll see that everything appears as it should at 50mm, not 80mm (where everything would be zoomed "in" a little bit and larger). You're just missing some crap around the edges. That's all.

[HakSpeame]

...crop factor ****...

..and so the crusade continues! [surrender]

[b3JOkwXL]

..and so the crusade continues! [surrender]

Well, I really have nothing better to do with my time, so....

[HedgeYourBets]

I just want to point out (since it's a pet peeve of mine) that crop factor does NOT change the focal length of a lens!!! A 50mm lens on a 1.6x crop camera is still taking photos at 50mm. It simply cuts the edges of the photo out. There's this weird misconception out there that cropping is identical to having a lens with a higher focal length, and it's simply not true.

If you dont believe me, put a zoom lens on your 1.6x crop body and zoom to 30mm. According to some people, this should make the lens an effective 50mm. But if you look through the viewfinder, you'll see that the field of view is zoomed out and objects appear smaller than they do with your naked eye (and 50mm is supposed to give the same field of view as your naked eye). Now change your zoom to 50mm, and you'll see that everything appears as it should at 50mm, not 80mm (where everything would be zoomed "in" a little bit and larger). You're just missing some crap around the edges. That's all.

I used to think that before. But the perspective you see of a lens has to do with the distance you are away from the object, rather than how much magnification you see.

So if you used a 50mm on a crop, you'd have to move the same distance as if you were using an 80mm lens on FF to get the same frame. Therefore, this same distance would give you the same perspective.

[teridbruse]

I used to think that before. But the perspective you see of a lens has to do with the distance you are away from the object, rather than how much magnification you see.

So if you used a 50mm on a crop, you'd have to move the same distance as if you were using an 80mm lens on FF to get the same frame. Therefore, this same distance would give you the same perspective.

Pretend you have 2 cameras with identical zoom lenses. One is a full frame camera, and the other is a 1.6x crop.

If you set both of the lenses to 50mm, aim the cameras at a nearby houseplant, and look through the viewfinder, the images will be identical except for one difference—the full frame camera will allow a little bit more of the image to be visible around the border. The perspective will be identical in both (Both cameras will duplicate the perspective of a normal lens and will match that of the human eye).

Now, if you remain seated in the same place, you'll need to zoom OUT the lens on the crop body to fill the same area in the viewfinder as you see on the full frame (or you could stand up and walk away). Either way, this means that the perspective will be different between the photos on the full frame and crop cameras, even though the image will fill up the same amount of area in the final print. Since the perspective isn't identical, it proves that the crop body camera does not gain a true increase in focal length.

[DF9sLGSU]

You know, if you have around $800 dollars, get a something like a Sigma 70-200mm f/2.8. I used to own it, and was very pleased with what it produced. 70-200mm is a very good range for general work, as well as portraiture when you stay in the bottom half of the range (70-135mm), which is perfect for this kind of shooting. And the relatively fast f/2.8 aperture will provide a good bokeh from a longer range.
I you can afford that, then that lens would be a very good purchase.
Here's a picture I took with it:




And 100% crops.






PS
The lens itself is also gorgeous.

Would you have any idea how that would compare with the Nikon 80-200mm f/2.8 D ED Zoom AF Lens 80-200 f2.8, both seem to have exactly the same specs, jsut that the Nikon is more expensive but then it is a Nikon.