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Lenses and focal leangth.

johnboy-22
Posted 12/05/2012 - 16:22 Link
I have a K5 with the following lenses. DAL 18-55. DAL 50-200, DA 18-135, M 100 macro and a M 135.

I have just bought the M 100 and was wanting to compare it with the 18-135 so set up the tripod and went to take some pictures. I then used the M100 and changed the lens to the 18-135 to discover that even at 135 the golf ball did not fill the picture. Stick in the M135 and discover that I had to use the 50-200 to get the same picture set at 200.
I thought the focal length did not change if all lenses were on the same body.
Pentax K5, Kx, DA15 ltd,18-135, 18-55, 50-200, M100 f4 macro, M135 f3.5 A28 F2.5
droopsnoot
Posted 12/05/2012 - 18:48 Link
There's a difference in effective focal length between 35mm and DSLR cameras, so if your M100 and M135 lens are originally intended for a film SLR, you need to multiple the focal length by 1.5 to get the figure as your K5 will see it, so they'll be 150mm and 202mm respectively. It's explained in the front of the Kx manual, probably has something in the K5 manual too.
Real name: Mike Edwards. My homage to seventies Vauxhalls: www.firenza.net

Camera - Pentax Kx, 18-55 kit lens, 18-200 Sigma, 50-500 Sigma, 500mm Tamron mirror
Edited by droopsnoot: 12/05/2012 - 18:55
droopsnoot
Posted 12/05/2012 - 18:57 Link
Maybe the "D" designation means that the focal length specified is based on a digital sensor size (which is what makes the difference). I've always wondered whether my DAL18-55 is actually equivalent to a film 18-55, or a film 27-83 lens.
Real name: Mike Edwards. My homage to seventies Vauxhalls: www.firenza.net

Camera - Pentax Kx, 18-55 kit lens, 18-200 Sigma, 50-500 Sigma, 500mm Tamron mirror
fatspider
Posted 12/05/2012 - 19:11 Link
Focal length doesn't change regardless of what format a lens is made for, a 50mm lens is still a 50mm lens whether its made for 135 or APS_c, as long as the lenses are used on the same format they should give similar results, obviously taking into account the crop factor on APS-c

In this case even if the M100 was made for a film body it's still a 100mm lens and should still cover the same "frame" as the 18-135 set to 100mm, even the 50-200 should give the same sized image when set a 100mm.

Having said that there may be some slight discrepancies where lenses are "alleged" to be a certain focal length but may fall short, but certainly not by another 100mm.

Either your lenses are faulty and defying the laws of physics or you've done something wrong somewhere.
My Names Alan, and I'm a lensaholic.
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JAK
Posted 12/05/2012 - 21:01 Link
The M100 should give more or less the same coverage as both the 18-135 and 50-200 when set at 100, providing the camera and golf ball remain at the same distance. So not sure what you're experiencing there.

You haven't by chance got a 2x teleconverter on the M100 ?

John
John K
Posted 12/05/2012 - 21:16 Link
Alan is right.
A lens focal length is the focal length. This does not change.
If you look at compacts they will state 5-15mm as the lens, even if you actually see a more narrow angle of view for the image.

Take for example the 50mm. A lens which was very common in 135 Film days.

On 135 film (or more commonly known as 35mm) you get the following
Full Frame digital (35mm on film) - 39.6 degrees
APS-C - 26.6 degrees (equiv on FF would be a 76.3mm lens)
"Pro compact - 1/1.7" " 8.7 degrees (equiv on FF would be a 236.8mm Lens)
cheap point and shoot (1/2.5") 6.6 Degrees (eqv on FF would be 312.5mm)
Medium format - 53.8 degrees. (eqv on FF would be 35.5mm)
Remember, it is STILL a 50mm Lens and will act as such.

The confusion is that the brands marketing confuse buyers with their "35mm equiv" or at worst with Ricoh, stating the equiv lens range in all their promotions, when in fact this is wrong, the lens is not a 50mm at all. But that is what the market want, as it easier for people to understand the 35mm version, rather than the angle of view.

So, to sum up, if on a lens, it states the focal length, it will be the focal length, it is up to you to work out the angle of view of said lens.

Hope this helps.
Some Cameras
Edited by Offertonhatter: 12/05/2012 - 21:18
johnboy-22
Posted 12/05/2012 - 23:04 Link
Ok I have taken 3 pictures of a fridge butterfly and cropped the butterfly in light room so that only it was in the image.

18-135 @135 was 1383 x 807 pixels
55-200@ 137.5 was 1993 x 1158 pixels
135 was 2171 x 1276 pixels

Now allowing for not too accurate cropping I would assume they should all be roughly the same.
Pentax K5, Kx, DA15 ltd,18-135, 18-55, 50-200, M100 f4 macro, M135 f3.5 A28 F2.5
johnboy-22
Posted 12/05/2012 - 23:11 Link
On the first 2 the pictures they looked the same in the viewfinder but on the 3rd you could see more of the fridge.
Pentax K5, Kx, DA15 ltd,18-135, 18-55, 50-200, M100 f4 macro, M135 f3.5 A28 F2.5
johnriley
Posted 13/05/2012 - 00:24 Link
You need to satisfy yourself that a 135mm lens will be just that, regardless of which lens it is.

Tripod is essential. Try the 18-135mm at 135mm and the 135mm for starters. Shoot the images, but equally well just look through the viewfinder. What you see will be the same in both cases.

Choosing 135mm on the 50-200mm is a bit more tricky as the lens markings may not be accurate enough. Also, the first two lenses may not be exactly 135mm, but should be close enough for this test.
Best regards, John
JAK
Posted 13/05/2012 - 00:28 Link
johnboy-22 wrote:
Ok I have taken 3 pictures of a fridge butterfly and cropped the butterfly in light room so that only it was in the image.

18-135 @135 was 1383 x 807 pixels
55-200@ 137.5 was 1993 x 1158 pixels
135 was 2171 x 1276 pixels

Now allowing for not too accurate cropping I would assume they should all be roughly the same.

How can you be certain that the zooms were both set to 135mm? Those markings on the lenses are only approximate.
To see if they were at 135mm, playback each of the images in the camera and press the info button until you see the more detailed info screen; it's shown there, or get it from the EXIF data in Lightroom.

John
John K
fatspider
Posted 13/05/2012 - 00:33 Link
Was the camera on a tripod or at least in the EXACT same position with each shot?
For tests and comparisons like this you need the camera in a fixed position, you then need to resize the images and post them on here for further feedback.
When I say resize I don't mean crop so they look similar, each image should be left as it was out of camera and simply resized to say 800 pixels wide.

I should also point out that the focal length markings on Zoom lenses are only intended as a guide and should not be taken as exact settings.

EDIT, damn, someone beat me to the enter key
My Names Alan, and I'm a lensaholic.
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Edited by fatspider: 13/05/2012 - 00:34
johnboy-22
Posted 13/05/2012 - 08:17 Link
I used a tripod and checked the exif data to give me the details. The 18-135 was set to max zoom. I will need to figure out how to post pictures when I get home tonight.

All I can say is that when I used the M100 lens it was zoomed in closer than the 18-135.

Thanks

John
Pentax K5, Kx, DA15 ltd,18-135, 18-55, 50-200, M100 f4 macro, M135 f3.5 A28 F2.5
Algernon
Posted 13/05/2012 - 08:28 Link
Lens FL's refer to infinity focus. They DO change at closer distances.
Internal focus lenses can change by a massive amount.

For lens testing I usually put two small (50mm long) pieces of electrical tape 500mm apart horizontally or two blobs of Blu-Tak and then move the camera backwards and forwards until the tape is just visible at the edge of the viewfinder. Repeat for the other lenses.

A few postage stamps or non-uk banknotes, wrist watch, piece of fabric will do for the test.
Half Man... Half Pentax ... Half Cucumber

Pentax K-1 + K-5 and some other stuff

Algi
Algernon
Posted 13/05/2012 - 09:55 Link
I forgot to mention:

Focusing is a big problem on tests, the adjustment for MF is too coarse and the 3" LCD hasn't got enough resolution. The simplest way is to take at least 4 shots and pick the best using a viewer that allows 1:1 comparisons.

A better way is to use a focusing rack, lock the lens focus with a piece of masking tape after focusing manually using LV. Take shots at 20mm and 40mm either side of the zero position as well as the zero position.

If you want to be a bit more precise find the best shot and then repeat +/- 10mm either side of it.
Half Man... Half Pentax ... Half Cucumber

Pentax K-1 + K-5 and some other stuff

Algi
johnboy-22
Posted 13/05/2012 - 16:37 Link
M100 macro lens
Comment Image

DA18-135@100mm
Comment Image

The above were taken from the same spot on a tripod.
M135 lens
Comment Image

DA18-135@135mm
Comment Image

The 2 above were also taken from a tripod at the same distance.
Pentax K5, Kx, DA15 ltd,18-135, 18-55, 50-200, M100 f4 macro, M135 f3.5 A28 F2.5

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