crop factor again!!

adwb · 11-27-2010, 02:12 AM

ok so the Pentax crop factor is 1.5 so the the 18-55 kit lens on 35mm camera now gives us the film equavelant to 27 to 82.5 on a Pentax digital, but what effect has the addition of a 2x converter? Does it just do that 2x magnification or is it changed?
I ought to know I suddenly have doubts to he answer to this based on some exif data I was looking at.

pop4 · 11-27-2010, 03:06 AM

The 18-55mm kit lens gives a Field of View equivalent of 27-82.5mm on a 35mm sensor. So the maths there seems right.

Add a 2x teleconverter into the mix, and the 18-55mm kit lens results in a lens that gives a focal length of 36-110mm. The FOV equivalent on a 35mm sensor will be 54-165mm.

jolepp · 11-27-2010, 01:46 PM

I'd guess that with most teleconverters with electrical contacts the body would use the original focal length (for everything, including exif) of the lens (if the lens can provide that) as the contacts are simply passed through. The Pentax ones might be different?

Class A · 11-27-2010, 01:53 PM

Unfortunately, the Pentax TCs are not different. Hence the EXIF data will be wrong.

Super A-wesome · 11-27-2010, 11:28 PM

May I ask, you arent planning to use that TC with the 18-55, because that would give you a painfully slow 36-110mm (54-165mm eq) f/7-11. remember, you lose two stops with a 2x TC, and on top of that, the sharpness will degrade.

And, if you aren't, then never mind

adwb · 11-28-2010, 03:33 AM

Originally posted by Super A-wesome

May I ask, you arent planning to use that TC with the 18-55, because that would give you a painfully slow 36-110mm (54-165mm eq) f/7-11. remember, you lose two stops with a 2x TC, and on top of that, the sharpness will degrade.

And, if you aren't, then never mind

correct but also bear in mind with the ever faster iso and now usable higher numbers the stops don't matter as much.
As far as degradation, first I have pentax tc with contacts so the glass is good, and for most of my shots it is on a 30mm lens and it's only the centre I want and that is almost always sharp.

alohadave · 11-28-2010, 01:44 PM

Originally posted by adwb

correct but also bear in mind with the ever faster iso and now usable higher numbers the stops don't matter as much.
As far as degradation, first I have pentax tc with contacts so the glass is good, and for most of my shots it is on a 30mm lens and it's only the centre I want and that is almost always sharp.

The higher ISO will help you get the exposure, but it won't help with Auto Focus. The AF sensor doesn't really work past about f/8ish.

RioRico · 12-04-2010, 06:56 PM

Back in the day, when I first put a Spiratone 400/5.6 on my 135/HF (half-frame) Olympus Pen-FT, I thought, "Oh wow, it's like a 600mm lens!" But I quickly learned that it wasn't. It was just a 400mm lens on a 18x24mm frame that cut off 1/4 of the image from each side. (Most 135 cams are landscape mode; the Pen-FT is portrait mode.)

When I backed that 400/5.6 with a 2x TC, I knew better than to think that it had become a 1200mm monster. Wide open, it would be an 800/11. The formatfaktor would give a FOV (field of view) like a slow 1200mm, but it's still only 800mm with the image edges sliced off. There are no free gifts in optics.

11-27-2010, 02:12 AM	#1
adwb Veteran Member Join Date: Sep 2009 Location: Bristol UK Photos: Gallery Posts: 1,636	crop factor again!! ok so the Pentax crop factor is 1.5 so the the 18-55 kit lens on 35mm camera now gives us the film equavelant to 27 to 82.5 on a Pentax digital, but what effect has the addition of a 2x converter? Does it just do that 2x magnification or is it changed? I ought to know I suddenly have doubts to he answer to this based on some exif data I was looking at.

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11-27-2010, 03:06 AM	#2
pop4 Veteran Member Join Date: Aug 2007 Location: YMML Photos: Gallery Posts: 1,879	The 18-55mm kit lens gives a Field of View equivalent of 27-82.5mm on a 35mm sensor. So the maths there seems right. Add a 2x teleconverter into the mix, and the 18-55mm kit lens results in a lens that gives a focal length of 36-110mm. The FOV equivalent on a 35mm sensor will be 54-165mm.

11-27-2010, 01:46 PM	#3
jolepp Veteran Member Join Date: Oct 2010 Location: Finland Photos: Albums Posts: 3,196	I'd guess that with most teleconverters with electrical contacts the body would use the original focal length (for everything, including exif) of the lens (if the lens can provide that) as the contacts are simply passed through. The Pentax ones might be different?

11-27-2010, 01:53 PM	#4
Class A Pentaxian Join Date: Aug 2008 Location: Wellington, New Zealand Posts: 11,251	Unfortunately, the Pentax TCs are not different. Hence the EXIF data will be wrong.

11-27-2010, 11:28 PM	#5
Super A-wesome Forum Member Join Date: Sep 2010 Posts: 94	May I ask, you arent planning to use that TC with the 18-55, because that would give you a painfully slow 36-110mm (54-165mm eq) f/7-11. remember, you lose two stops with a 2x TC, and on top of that, the sharpness will degrade. And, if you aren't, then never mind

11-28-2010, 03:33 AM	#6
adwb Veteran Member Join Date: Sep 2009 Location: Bristol UK Photos: Gallery Posts: 1,636 Original Poster	Originally posted by Super A-wesome May I ask, you arent planning to use that TC with the 18-55, because that would give you a painfully slow 36-110mm (54-165mm eq) f/7-11. remember, you lose two stops with a 2x TC, and on top of that, the sharpness will degrade. And, if you aren't, then never mind correct but also bear in mind with the ever faster iso and now usable higher numbers the stops don't matter as much. As far as degradation, first I have pentax tc with contacts so the glass is good, and for most of my shots it is on a 30mm lens and it's only the centre I want and that is almost always sharp.

11-28-2010, 01:44 PM	#7
alohadave Veteran Member Join Date: Jan 2008 Location: Quincy, MA Photos: Gallery \| Albums Posts: 2,024	Originally posted by adwb correct but also bear in mind with the ever faster iso and now usable higher numbers the stops don't matter as much. As far as degradation, first I have pentax tc with contacts so the glass is good, and for most of my shots it is on a 30mm lens and it's only the centre I want and that is almost always sharp. The higher ISO will help you get the exposure, but it won't help with Auto Focus. The AF sensor doesn't really work past about f/8ish.

12-04-2010, 06:56 PM	#8
RioRico Veteran Member Join Date: Jun 2008 Location: Limbo, California Posts: 11,263	Back in the day, when I first put a Spiratone 400/5.6 on my 135/HF (half-frame) Olympus Pen-FT, I thought, "Oh wow, it's like a 600mm lens!" But I quickly learned that it wasn't. It was just a 400mm lens on a 18x24mm frame that cut off 1/4 of the image from each side. (Most 135 cams are landscape mode; the Pen-FT is portrait mode.) When I backed that 400/5.6 with a 2x TC, I knew better than to think that it had become a 1200mm monster. Wide open, it would be an 800/11. The formatfaktor would give a FOV (field of view) like a slow 1200mm, but it's still only 800mm with the image edges sliced off. There are no free gifts in optics.