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05-27-2014, 11:03 AM   #1
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18-55 dslr equivilant

I am interested in using my new k5iis for some astrophotography. Milky way and meteor shots and the like.
I am planning on purchasing the DA 18-55 wr in a few months. In the mean time I have an older SLR lens < SMC Pentax FA 1:3.5-5.6 28-80mm>
How does this lens translate in mm back to DSLR from SLR. I understand its not a fixed figure because of sensor sizes, but would it be somewhat close to an 18-55?

Thanks for any help.

DeWolf

05-27-2014, 11:21 AM   #2
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It remains a 28-80mm lens. Focal length does not change when you change bodies. That being said, you can expect a significantly (2/3) narrower field of view (FOV) at all focal lengths than with 35mm film. On APS-C (your K-5IIs), your 28-80 lens is a wide-normal to short tele. Of course, you could just attach the lens to the camera and see whether the FOV is appropriate for your needs.


Steve

(...proud to have not mentioned crop factor once...)

Last edited by stevebrot; 05-27-2014 at 11:27 AM.
05-27-2014, 11:23 AM   #3
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The "dslr equivalent" business is rather confusing. First off, a mm is a mm. The focal length is a property of the lens and will never change, no matter what camera you mount it on. What type of camera it was designed for doesn't matter. (Just as Steve just said )

The "equivalent" comes in when you try to compare what the field of view for a lens is when mounted on cameras with different sensor size. Which means that the 28-80 will "look" the same on a Pentax DSLR as a 42-120 would on a 35mm camera (if such a thing had existed).

(Umm, and I didn't mention crop factor either - at least not directly...)
05-27-2014, 11:24 AM   #4
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For the equivalent field of view on a film camera multiply what you have currently by 1.5. Your 28-80 gives roughly the same field of view on a flim camera as an 18-55 gives on an APS-C DSLR like your K5iis.

While the mm of the lenses are the same regardless of format used your 28-80 would give a 35mm field of view equivalent of 42-120, meaning it's not going to be as wide of an angle as you seem to want.

05-27-2014, 02:02 PM   #5
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Crop factor. There, I said it.

Seriously, what the other guys said. A 28-80 on any Pentax DSLR is going to give you a 50% "longer" field of view than on a 135 (full frame) film camera. Whether or not this focal length will be usable for astrophotography I am not sure as I have never attempted it.
05-27-2014, 06:19 PM   #6
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QuoteOriginally posted by cbope Quote
Crop factor. There, I said it.
Slap your hand multiple times for perpetuating a meaningless and confusing term.


Steve

(...just kidding of course...the term is not quite meaningless, but it seems very few people understanding cropping...)
05-27-2014, 08:56 PM   #7
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QuoteOriginally posted by VoiceOfReason Quote
For the equivalent field of view on a film camera multiply what you have currently by 1.5. Your 28-80 gives roughly the same field of view on a flim camera as an 18-55 gives on an APS-C DSLR like your K5iis.

While the mm of the lenses are the same regardless of format used your 28-80 would give a 35mm field of view equivalent of 42-120, meaning it's not going to be as wide of an angle as you seem to want.
ah, and there is the real answer I was looking for, " not going to be as wide of an angle as you seem to want.
Yes I was confused as to focal length and field of view. I want something wider than the 28-80 allows. So I will be ordering the DA 18-55 WR soon, unless someone has a better suggestion for around the same price.

Thanks again for your prompt and wonderful help. I am Very happy with the camera and the DA 55-300 that was recommended here. Much more detail and sharper than the previous setup.

DeWolf
05-27-2014, 09:26 PM   #8
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The 18-55 WR is a decent lens, you will need to spend at least double to get something better, like the Sigma 17-50 f/2.8 or Tamron 17-50 f/2.8.

05-27-2014, 09:39 PM   #9
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Is there any other reason outside of price that you're focused on the 18-55? Is it the WR status? The F3.5 at the wide end won't do you many favors for astrophotography. I might consider something like the Sigma 17-70 which at least gives you F2.8 at the wide and still F4 at the tele end of the range. Mine does have some coma in the corners but I think it would be a better overall lens than any of the entry Pentax 18-55 lenses.
05-27-2014, 10:34 PM   #10
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QuoteOriginally posted by cbope Quote
The 18-55 WR is a decent lens, you will need to spend at least double to get something better, like the Sigma 17-50 f/2.8 or Tamron 17-50 f/2.8.
At least double. The Tamron is currently at about $500 USD with the Sigma at $569 USD. I own and can recommend the Sigma 17-70/2.8-4 (C) and got it on sale for $449 USD. I have not used it for astrophotography though


Steve
05-28-2014, 04:55 AM   #11
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QuoteOriginally posted by stevebrot Quote
At least double. The Tamron is currently at about $500 USD with the Sigma at $569 USD. I own and can recommend the Sigma 17-70/2.8-4 (C) and got it on sale for $449 USD. I have not used it for astrophotography though


Steve
Thank you, I will look into both lenses, although it will take more time to save up for them.

DeWolf
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