Confused on equivalent focal length

Hi,

I'm relatively new to the forum and have what could be a silly question.

Although I've had my Pentax K-x for a while now, I'm only now looking to take advantage of all its benefits. So in the spirit of learning I picked up a 2.5/135mm Takumar Bayonet prime lens on the weekend to have a go at a fully manual lens. Prior to purchasing the lens I knew to expect that due to the APS-C crop factor the 135mm Takumar would give an equivalent focal length somewhere jsut above 200mm.

In toying around with the lens I was looking to compare the image quality with my Pentax 50-200mm kit lens and found the 135mm Takumar to take photos with an equivalent field of view to the Pentax 50-200mm when the latter was set to 135mm of zoom.

Am I missing something? Was I wrong to expect the Takumar to have a smaller field (greater focal length) of view than the pentax set at 135mm?

Why would it? You use both lenses on the same crop body so 135mm on the zoom will give you the same FOV of the 135mm on the Takumar. Why would you expect the Takumar to have smaller field than the 50-200 on an equivalent setting?

Well, as I understood it, the 50-200mm kit lens is a DA lens, thus made for the APS-C and therefore is uncropped. Whereas the Takumar is 135mm fullframe lens thus, if you applied the crop factor of 1.52, it should be equivalent to an APS-C lens focal length of 205mm.

The focal length stated on lenses is always the actual focal length that you would get on a FF camera. So the 135 when put on a crop sensor camera would give you a 205 equivalent focal length. The 50-200 would give you a 75-300equivalent focal length.

The DA designation does not mean that the focal length is stated after being adjusted for a cropped sensor, it rather means that if you try the lens on a full frame / film camera (assuming you can) it would provide plenty of vignetting as the lens does not cover adequately the larger area of the film/ff sensor. However it will then provide the stated FOV.

I hope I haven't confused you.

That does make sense actually. I didn't know that the focal length on DA lenses is stated in FF terms.

Thanks for your help

Focal length is focal length no matter the format. You will notice compact cameras do state actual focal length on them as well. A 200mm is a 200mm because the light path is 200mm. Equivalent focal length is really made up nonsense that causes confusion, equivalent field of view would be more accurate.

Take off the full frame camera. The focal length is the distance between the nodal point and the sensor/film plane at infinity. It means that a 135mm lens is always 135 mm, be it on a APS-C, 35mm or medium format camera. Actually, "full frame" is a misnomer. APS-C is full frame, so is 35 mm and medium format.

Forget about full frame and "crap factor". Just remember, on APS-C, 30 mm is more or less a standard lens. Longer than that is telephoto and shorter is wide angle.

If you ever use 24 X 36 format cameras, then 50 mm is standard. Longer is telephoto and shorter is wide angle.

On medium format, 75 mm is normal...

Amen to that

Continually thinking about what equivalent something is going to give will only tax the brain cells, unless your going to be using different formats there's no need for it, just accept that a given focal length will produce a given FOV on whatever format your using.

Having learnt photography on 35mm flim, I find the crop factor quite helpful. Especially when it comes to comparing different bodies. But I can't see why it was ever described as effective focal length, that has caused so much confusion!
The crop factor is a function of the camera (sensor) whilst focal length is an intrinsic function of the lens. I've seen some e-bay sellers claiming fantasic focal lengths for lenses by applying the crop factor, that just screems to me "stear clear, con artist at work". I'd accept it as confusuion if it was from a obvious novice but it's usually traders...

Why?

You just have to think in relation to the format you use, not the one you used to use.

I work with medium format and APS-C and never use the "crap factor". I know what lens gives what field of view on each format, and go with that.

On APS-C, I know 30 mm is "normal", so everything longer is tele, and everything shorter is wide. It's that simple.

Your approach is more or less like someone who tries to learn a second language, and doesn't want to associate the foreign words with the idea or object they represent, but to the words they represent in the persons native language. On the long run, it slows down the process so much that it makes it almost unworkable.

It's quite handy to have a yardstick for comparing across systems, especially if you have limited experience of them. 35mm equivalent focal length seems a sane starting point - hence you see point and shoot cameras with tiny sensors marketed as '28-200mm lenses' (meaning they provide an angle of view equivalent to those focal lengths on 35mm film). If I was contemplating moving to medium format, I would also probably appreciate knowing the angle of view I was to expect from a given lens, and the easiest way of doing that is to state what focal length would provide the same angle of view on 35mm.

Having used APSC cameras for a while I am still not quite as used to what focal lengths look like with different lenses, compared to 35mm cameras. It's a bit like thinking of distance in km vs miles; it just involves a quick mental adjustment....

I've now learnt the 'second language' so that when I use one of my old 35mm cameras I end up using the crop factor in reverse... But more often it helps when I'm on forums where some members use micro 4/3, CSC, medium format...

In addition longer than 30 is tele, doesn't help much when the choice s 55-300mm, 600mm or 1000mm.
I use lenses from 10mm to 1000mm just knowing normal is less knowing the language than my knowledge of French!!

There is no problem there. Forget the last digit on your zoom or tele, and divide by three, and you have your enlargement (reduction) ratio right there. Couldn't be simpler.

Beside, some "new generation" photographers never worked with 35 mm cameras, so why confuse them right there.

Your suggestion is that each time I use a lens I do a 'enlargement (reduction) ratio' calculation based on remebering the normal focal length for that body. Is that easier than knowing the magnification with this body is 1.5* what I was used to with film? I'd also find it difficult to visulise a '10* enlargement ratio', where thinking '300mm is about right for seeing that bit' is more in keeping with my experience.

As I said before it's the 'effective focal length' that has caused confusion. The crop factor should never have been related to lenses, it's a function of the body, and is of significance if I'm after a new camera.

These days it is easy to mount K series lenses on 35mm film, APS-C cameras (DSLRs & soon the K-01) and micro 4/3 cameras (with a readily available adapter), the Pentax Q will apparently be added to the list soon as an adapter is planned.

Knowing the crop factor of the cameras helps visualise the results each form factor would have with my lenses/potential lenses. Similar visulaisations are needed by 'new generation' photographers in comparing systems.

The 'Crop factor' you're deriding is very much like your 'enlargement (reduction) ratio' but only needs employing on camparing sensor formats not every time you change lens. If I had the money to go medium format, knowing the relative crop factor of 6*7, 645 & 645D... to the historical 35mm standard, would help me choose the right version & the relevant focal lengths.

When the crop factor was first used practically all photographers were familar with 35mm - the exceptions having probably only used compact cameras. If it were to be invented today would a different reference format make more sense? I think not as the most popular size APS-C has several varieties (crop factors around 1.5 or 1.6).

It is easier, since most people will divide the focal length of their lens by "50" to get the enlargement ratio on so called full frame, then multiply that factor by 1.5 to get the corresponding ratio on APS-C.

I always worked with more than one format, and, before APS-C format in digital cameras, photographers (me included) never used the so called crop factor. It just doesn't make sense.

I'd also be surprised if Womble used a "crop factor" when he takes pictures with a large format camera.