Old 300mm equals what?

djayvo · 06-02-2009, 05:51 PM

I thought I had this figured out. My old 300mm lens I was told and thought I read was equal to around a 450mm on a DSLR. I am interested in picking up a new DA*300 and they say it equals to a 450mm 35mm. This would make my old lens less than 300mm on a DSLR. If some one could explain that would be appreciated.

**Marc Sabatella** · 06-02-2009, 06:01 PM

A 300mm lens is a 300mm lens, period, whether or old or new. So you old 30mm and a new DA*300mm are identical in this respect. When mounted to a DSLR with a 1.5X crop factor, either lens will produce a field of view that is similar to a 450mm lens mounted to a film camera. The change in FOV comes about not because of the age of the lens, but in the size of the imager (film or sensor) in the camera. *Any* lens mounted to a DSLR with a 1.5X crop factor (eg, all Pentax DSLR's) will produce a narrower FOV than it will on a 35mm film camera - it will produce the same FOV on digital as a lens 1.5 times as long would on film.

So if you own a 35mm film camera and use it regularly and have a good idea what focal lengths produce what fields of view, this is good info. If you don't use film cameras regularly, you can forget you ever heard about a crop factor - it's *only* relevant when comparing one lens on different cameras, not when comparing different lenses on one camera.

*sinus007* · 06-02-2009, 06:07 PM

Hi djayvo,
your old lens with focal length (FL) of 300mm will be the same FL no matter what camera you put it on. It will be the same (almost) as DA*300. IOW, the FL is always the same, field of view changes depending on the size of the sensor.
There're many posts on this topic here and other photo forums. Also, you can take a look at Field of view - Wikipedia, the free encyclopedia
Hope it helps.

Lowell Goudge · 06-02-2009, 06:08 PM

300 mm is 300 mm as marc has said. Perhaps if you can't relate to what a 450mm lens would have given you on a film camera in terms of filling the frame, then perhaps you can compensate this "lack of experience" with a little math.

for distant subjects,

Image size = Subject size x Focal length / distance

Keep all units in meters and remember that your sensor is .024 x .016 meters (w x h)

if you have a particular subject in mind, at a typical distance you can calculate how much of your fiew finder it will fill.

djayvo · 06-04-2009, 05:08 PM

Thanks Marc Sabatella for answering without any cynicism I have read maybe to many forums and posts on this and that is what got my thinking mixed up. Just needed someone re-pointing me in the right direction again.

ILoveVerdi · 06-04-2009, 05:37 PM

Originally Posted by djayvo

Thanks Marc Sabatella for answering without any cynicism I have read maybe to many forums and posts on this and that is what got my thinking mixed up. Just needed someone re-pointing me in the right direction again.

The key thing to remember is that the focal length is a property of the lens itself. The lens will bend light at the same angles whether the sensor is full frame, crop, or your eye. The whole "equivalency" thing is confusing, because it's mostly for those photographers who are used to the image size of film. It's a context held over (I'm guessing) from the days when digital was young and film still held sway (some call those the halcyon days), and full-frame sensors were a silicon manufacturer's nightmare. I guess it's a decent context to keep, as inconvenient as it may be for the moment: I really do think that in 10-20 years all D-SLRs will be full frame, which will bring us round-about to 35mm again. (Yes, even Olympus. Then we can convince them to make Zukikos in a K-mount.)

I just looked this up, wikipedia has a great article on the subject: Focal length - Wikipedia, the free encyclopedia (warning, past the first paragraph there will be math)

jogiba · 06-04-2009, 06:28 PM

Originally Posted by djayvo

I thought I had this figured out. My old 300mm lens I was told and thought I read was equal to around a 450mm on a DSLR. I am interested in picking up a new DA*300 and they say it equals to a 450mm 35mm. This would make my old lens less than 300mm on a DSLR. If some one could explain that would be appreciated.

\
How did YOU end up thinking that ? Both 300mm lenses have the same FL. With APS-C sensor just multiply and FL by 1.5.

djayvo · 06-04-2009, 08:53 PM

Hi jobiba. I checked posts and talked to the camera sales person who basically showed me that my old 300mm lens I bought new 35 yrs ago was comparable to a 450mm due to the sensor size this sparking an interest in Pentax again. This I understood and liked for my old lens gave the appearance of a longer reach and some of the edging problems I had on my film camera seem to be gone with the DSLR.
Now looking to get something nice I went on the Pentax site to check out the DA 300mm and just misread the info they supplied twice “note to self after a 14hr shift go to bed

)

Anyways after getting it straight in my head again I re-read the statement and could follow it. Just a little dyslexic that day I suppose.

*OregonJim* · 06-04-2009, 10:51 PM

Originally Posted by djayvo

Anyways after getting it straight in my head again I re-read the statement and could follow it. Just a little dyslexic that day I suppose.

Djayvo, you're perfectly normal. I think we all had to think twice when first exposed to the 'crop factor' phenomenon and the mental math gyrations that followed...

bymy141 · 06-05-2009, 08:02 AM

Tamron has a very cool online tool on their website: Focal length comparison tool, Tamron USA
With the slider under in the window you can zoom in and see the length effect.
Also there are 2 knobs: 35 mm and Digital seeing what the effect is of 35 mm (or Full Frame) and APS-C camera's.

Of course APS-C lenses cannot be used for 35 mm since they work with smaller projection surface

Have fun playing....

- Bert

wildman · 06-05-2009, 01:21 PM

Originally Posted by djayvo

I thought I had this figured out. My old 300mm lens I was told and thought I read was equal to around a 450mm on a DSLR. I am interested in picking up a new DA*300 and they say it equals to a 450mm 35mm. This would make my old lens less than 300mm on a DSLR. If some one could explain that would be appreciated.

The most important practical consideration when using a long lens is apparent magnification.

So for every 100mm focal length you have about 3x of magnification in a APS-C camera thus:

300mm=3x3=9x.

or 500mm=3x5=15x etc.

It's quick and dirty and useful and you don't have to think about crop factor etc.

After all magnification is what you are using a tele for in the first place.