How do I pick a lens size?

713alan · 04-27-2007, 07:08 AM

I am new to SLR cameras. I have the K10D. I currently have two zoom lenses, a 16-45 and a 28-105. I know about zoom range (45/16, 105/28) of the lenses and the field of view crop factor of the sensor. I'm not really worried about that.

I've taken pictures at the maximum zoom on the 28-105 lens. Looking at the pictures, I would like more magnification. On my computer, lets say I magnify in Gimp and find I like the picture at 175X. What lens size would give me this shot? I understand I am trading field of view for magnification with the new lens.

I am guessing that it would just be 1.75 x 105 = 183 mm. Is that correct?

FotoPete · 04-27-2007, 08:07 AM

your dsLr will be recording the aproximate of 1.5X your lens. Further magnification is applied over top.

Say you shoot at 105mm. You photo will look like one that was shot with a 157.5mm lens. So if you maginify that image by 1.75 with your program. You'll get 105mm x 1.5 x 1.75.

mattdm · 04-27-2007, 08:18 AM

Originally posted by FotoPete

your dsLr will be recording the aproximate of 1.5X your lens. Further magnification is applied over top.

Say you shoot at 105mm. You photo will look like one that was shot with a 157.5mm lens. So if you maginify that image by 1.75 with your program. You'll get 105mm x 1.5 x 1.75.

I don't think the crop factor multiplier comes into play in the original poster's equation, because he's using the same camera all along. The 1.5× is only a meaningful number in comparison with 35mm film cameras.

So the answer to the original question is: yes, on the simplest level, that's all there is to it. This is why point-and-shoot digital cameras can offer a "digital zoom" feature, which simply crops out the middle and expands it in software just as you're doing on your computer -- exact same thing.

Cideway · 04-27-2007, 08:20 AM

OKay, i'll take a stab, i am not sure what you mean by 175x but with focal length it works out pretty much like this:

50mm is the "standard" focal length, similar magnification to what the eye sees.

24mm is roughly half the magnification of the 50.

100 is twice, the magnification

200 is four times and 300 is six times

every time you double the focal length the apparent size of an object in the picture doubles (all things being equal).

GaryML · 04-27-2007, 08:51 AM

Originally posted by 713alan

I am new to SLR cameras. I have the K10D. I currently have two zoom lenses, a 16-45 and a 28-105. I know about zoom range (45/16, 105/28) of the lenses and the field of view crop factor of the sensor. I'm not really worried about that.

I've taken pictures at the maximum zoom on the 28-105 lens. Looking at the pictures, I would like more magnification. On my computer, lets say I magnify in Gimp and find I like the picture at 175X. What lens size would give me this shot? I understand I am trading field of view for magnification with the new lens.

I am guessing that it would just be 1.75 x 105 = 183 mm. Is that correct?

Correct! If you want 1.75X the magnification of a 105mm lens, then you want a focal length of about 183mm.

Some lenses to consider:

1. If you want a zoom, then the Pentax DA 50-200mm f4-5.6 would be a good choice for a relatively inexpensive zoom. There are also Tamron, Sigma and other Pentax consumer zooms in this class (including 75-300mm lenses), but the DA 50-200 is probably the best performer.

If you want a wider aperture, then something like a used Pentax FA* 80-200 (hard to find and expensive) Sigma 70-200mm f2.8 (somewhat hard to find and somewhat expensive), or a Sigma 100-300 f4 (available new, but heavy and expensive) may be considered. I have the Sigma 100-300 f4 EX DG and it is an excellent lens. But it's 8 inches long, over 3 lbs, and priced around $1000.

2. If you want a new, fixed focal length lens, then you might wait for the Pentax DA* 200mm f2.8 that will be released some time later this year. It is expected to be a top performer, and it will have an in-lens focusing motor for fast & quiet focusing on the K10D. I'm guessing the price will be around $900-1200. Expected delivery is September 2007, but you should add 3 months to that based on Pentax' past performance. Sigma makes a 180mm f3.5 macro lens: good quality but pricey (around $900) and may be hard to find in stock. It would be a good choice for macro shots of insects and small animals. But for general use, I'd wait for the DA* 200 (and maybe get the DA 50-200 to use for the next 7 months).

3. There are a number of Pentax 200mm lenses made in the past that can be bought used. A SMC-M 200mm f4 is inexpensive and widely available. But metering with non-A lenses can be a problem with the K10D, and it is not auto-focus nor auto-exposure. Something like a Pentax A* or FA* 200mm f2.8 would give better results but they are much more expensive used (maybe even more than the DA* 200 will sell for new).

mattdm · 04-27-2007, 09:32 AM

Originally posted by GaryML

Correct! If you want 1.75X the magnification of a 105mm lens, then you want a focal length of about 183mm.
1. If you want a zoom, then the Pentax DA 50-200mm f4-5.6 would be a good choice for a relatively inexpensive zoom. There are also Tamron, Sigma and other Pentax consumer zooms in this class (including 75-300mm lenses), but the DA 50-200 is probably the best performer.

And if you're interested in this, think fast, because the $50 rebate ends at the end of April.

04-27-2007, 07:08 AM	#1
713alan Junior Member Join Date: Nov 2006 Location: Houston Posts: 46	How do I pick a lens size? I am new to SLR cameras. I have the K10D. I currently have two zoom lenses, a 16-45 and a 28-105. I know about zoom range (45/16, 105/28) of the lenses and the field of view crop factor of the sensor. I'm not really worried about that. I've taken pictures at the maximum zoom on the 28-105 lens. Looking at the pictures, I would like more magnification. On my computer, lets say I magnify in Gimp and find I like the picture at 175X. What lens size would give me this shot? I understand I am trading field of view for magnification with the new lens. I am guessing that it would just be 1.75 x 105 = 183 mm. Is that correct?

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04-27-2007, 08:07 AM	#2
FotoPete Veteran Member Join Date: Feb 2007 Location: Toronto Photos: Gallery Posts: 1,774	your dsLr will be recording the aproximate of 1.5X your lens. Further magnification is applied over top. Say you shoot at 105mm. You photo will look like one that was shot with a 157.5mm lens. So if you maginify that image by 1.75 with your program. You'll get 105mm x 1.5 x 1.75.

04-27-2007, 08:18 AM	#3
mattdm Veteran Member Join Date: Feb 2007 Location: Boston, MA Photos: Gallery Posts: 2,948	Originally posted by FotoPete your dsLr will be recording the aproximate of 1.5X your lens. Further magnification is applied over top. Say you shoot at 105mm. You photo will look like one that was shot with a 157.5mm lens. So if you maginify that image by 1.75 with your program. You'll get 105mm x 1.5 x 1.75. I don't think the crop factor multiplier comes into play in the original poster's equation, because he's using the same camera all along. The 1.5× is only a meaningful number in comparison with 35mm film cameras. So the answer to the original question is: yes, on the simplest level, that's all there is to it. This is why point-and-shoot digital cameras can offer a "digital zoom" feature, which simply crops out the middle and expands it in software just as you're doing on your computer -- exact same thing.

04-27-2007, 08:20 AM	#4
Cideway Inactive Account Join Date: Feb 2007 Location: Adelaide, South Australia, Australia Photos: Gallery Posts: 364	OKay, i'll take a stab, i am not sure what you mean by 175x but with focal length it works out pretty much like this: 50mm is the "standard" focal length, similar magnification to what the eye sees. 24mm is roughly half the magnification of the 50. 100 is twice, the magnification 200 is four times and 300 is six times every time you double the focal length the apparent size of an object in the picture doubles (all things being equal).

04-27-2007, 08:51 AM	#5
GaryML Inactive Account Join Date: Nov 2006 Location: San Diego, California Posts: 441	Originally posted by 713alan I am new to SLR cameras. I have the K10D. I currently have two zoom lenses, a 16-45 and a 28-105. I know about zoom range (45/16, 105/28) of the lenses and the field of view crop factor of the sensor. I'm not really worried about that. I've taken pictures at the maximum zoom on the 28-105 lens. Looking at the pictures, I would like more magnification. On my computer, lets say I magnify in Gimp and find I like the picture at 175X. What lens size would give me this shot? I understand I am trading field of view for magnification with the new lens. I am guessing that it would just be 1.75 x 105 = 183 mm. Is that correct? Correct! If you want 1.75X the magnification of a 105mm lens, then you want a focal length of about 183mm. Some lenses to consider: 1. If you want a zoom, then the Pentax DA 50-200mm f4-5.6 would be a good choice for a relatively inexpensive zoom. There are also Tamron, Sigma and other Pentax consumer zooms in this class (including 75-300mm lenses), but the DA 50-200 is probably the best performer. If you want a wider aperture, then something like a used Pentax FA* 80-200 (hard to find and expensive) Sigma 70-200mm f2.8 (somewhat hard to find and somewhat expensive), or a Sigma 100-300 f4 (available new, but heavy and expensive) may be considered. I have the Sigma 100-300 f4 EX DG and it is an excellent lens. But it's 8 inches long, over 3 lbs, and priced around $1000. 2. If you want a new, fixed focal length lens, then you might wait for the Pentax DA* 200mm f2.8 that will be released some time later this year. It is expected to be a top performer, and it will have an in-lens focusing motor for fast & quiet focusing on the K10D. I'm guessing the price will be around $900-1200. Expected delivery is September 2007, but you should add 3 months to that based on Pentax' past performance. Sigma makes a 180mm f3.5 macro lens: good quality but pricey (around $900) and may be hard to find in stock. It would be a good choice for macro shots of insects and small animals. But for general use, I'd wait for the DA* 200 (and maybe get the DA 50-200 to use for the next 7 months). 3. There are a number of Pentax 200mm lenses made in the past that can be bought used. A SMC-M 200mm f4 is inexpensive and widely available. But metering with non-A lenses can be a problem with the K10D, and it is not auto-focus nor auto-exposure. Something like a Pentax A* or FA* 200mm f2.8 would give better results but they are much more expensive used (maybe even more than the DA* 200 will sell for new).

04-27-2007, 09:32 AM	#6
mattdm Veteran Member Join Date: Feb 2007 Location: Boston, MA Photos: Gallery Posts: 2,948	Originally posted by GaryML Correct! If you want 1.75X the magnification of a 105mm lens, then you want a focal length of about 183mm. 1. If you want a zoom, then the Pentax DA 50-200mm f4-5.6 would be a good choice for a relatively inexpensive zoom. There are also Tamron, Sigma and other Pentax consumer zooms in this class (including 75-300mm lenses), but the DA 50-200 is probably the best performer. And if you're interested in this, think fast, because the $50 rebate ends at the end of April.