THINKING of buying a "primary" (telephoto?) lens for my Canon.

[Kym]

Peeps!!! Focal length is ALWAYS Focal length!

I made a pretty diagram to show it http://www.ausphotography.net.au/for...vs_Crop_Factor

Also example: http://www.ausphotography.net.au/for...ctor_w_example

So use the right terminology and don't use the mis-term 'multiplication' when all it's only a crop factor.

[Mr Felix]

Thanks guys.

I don't want to start WWIII here. ;-)

I can sort of "read between the lines" of what vk2gwk was saying.

There are many discussions about how to explain things to make them understandable to others without dumbing down the information.

Rather putting it in a way which is easier understood.

Though thanks for the explanation that the focal length doesn't change. Most/many people to whom I have spoken use the x1.6 analogy.

However, now I understand it isn't quite THAT simple. There are perspectives, and all that stuff which I haven't considered.

As I said though: I am only thinking about it at this point.

Money doesn't go as far nowadays and I guess like many: The bills seem to keep coming.

[arthurking83]

In theory you are absolutely correct, but for all practical purposes everyone uses the multiplication factors .....

Most reliable sources of information actually give an equivalent FOV value .. ie. as Xenedis says a crop factor value ... for a lens on the full frame 35mm format, not an actual multiplication factor of the lens.


The reason many of us prefer not to use any reference to lens multiplication or focal length multiplier, is more of a technical one, as already said .. for example that of DOF for a given lens and aperture value.

while I don't worship the DOF calculator site, it is a good indication of expectations of DOF for a given application:

Canon APS-C crop camera @ 35mm and 3m focus distance:

Subject distance 3 m

Depth of field
Near limit 2.77 m
Far limit 3.27 m
Total 0.49 m

In front of subject 0.23 m (46%)
Behind subject 0.27 m (54%)

Canon full frame @ 56mm and 3m focus distance(ie. supposedly for a similar FOV)

Subject distance 3 m

Depth of field
Near limit 2.86 m
Far limit 3.16 m
Total 0.3 m

In front of subject 0.14 m (47%)
Behind subject 0.16 m (53%)

Hyperfocal distance 58.7 m
Circle of confusion 0.03 mm

As you can see, while it may appear that the 35mm lens looks like a 56mm lens equivalent on the full frame camera, the reality is that it will actually look nothing like it as the DOF is much more shallow on the full frame camera.
Just one reason why focal length multiplier is not a good choice of wording as a substitute for crop factor.

For most of us, we understand the terminology and what is being described, but for someone looking for accurate info it's always best to give them the correct info to begin with.

[Xenedis]

As you can see, while it may appear that the 35mm lens looks like a 56mm lens equivalent on the full frame camera, the reality is that it will actually look nothing like it as the DOF is much more shallow on the full frame camera.

Yes, there is that issue, too, but I didn't want to go there. :-)

Essentially, all that's common between a 50mm lens on a 650D and an 80mm lens on a 5D II is the field of view. Even then, it's not precise, as the 5D's sensor is not exactly 1.6x the size of the 650D's sensor. Fun, innit?

For most of us, we understand the terminology and what is being described, but for someone looking for accurate info it's always best to give them the correct info to begin with.

Unfortunately, many people mistakenly think that placing a 50mm lens on a 650D turns it into an 80mm lens, and when I read or hear someone making a claim along those lines, I feel it needs to be corrected. There's too much mis-information floating around about this subject, and marketing hype in the form of erroneous terms like 'focal length multiplication factor' is largely to blame.

Without getting overly technical (as indeed this subject is when it comes to exact sensor size, variations between lenses with the same marked focal length, DOF and perspective, etc.), the simplest explanation we can give to people who don't understand the consequence of using an EF lens on an APS-C camera is that the field of view or framing is cropped.

[ameerat42]

Look! Forget all that's gone before. Where's Mr Felix?

[Xenedis]

Look! Forget all that's gone before. Where's Mr Felix?

He had probably had enough of all this APS-C and full-frame stuff, and went out to buy a medium-format camera system...

[ameerat42]

^ (It being the latest craze in laughs!)
Am.

[Mr Felix]

No, I'm still here.

Just trying to get my head around it all and process it so I am not simply emptying my pocket/s as someone suggested.

I sort of understand what is being said and how the FOV is changed rather than the focal length.

I posted a couple of thanks...

But I guess being "reminded" the real difference I do want to make sure I know what it is all about.


Again, thanks for the explanations.

I am also trying to organise my time for the Get together on Sat at middle head.

I am also trying to get my head around Broad Band.
As that is off topic, I won't go into the details. Suffice to say it is painful/confusing/annoying.

[Mark L]

........
I am also trying to get my head around Broad Band.
As that is off topic, I won't go into the details. Suffice to say it is painful/confusing/annoying.

reckon someone'll help if you go into details here ..... http://www.ausphotography.net.au/for...9-Out-Of-Focus

[Mr Felix]

Mark,

Thanks much for the offer, but I am at information overload just now as well.

[Kym]

I am also trying to get my head around Broad Band.
As that is off topic, I won't go into the details. Suffice to say it is painful/confusing/annoying.

Easy Broadband ... http://bc.whirlpool.net.au/
And http://www.internode.on.net/residential/adsl_broadband/

[Roosta]

Good day Mr Felix,

Not with standing some very valid and technical points here from William W and Xenedis (Who both know what their on about) I find this site to be a wealth of information. If you go there, then access the REVIEWS section top of page, then you can research the lenses you've mentioned.

There is also a section here Where I have preselected two of the lenses you've mentioned, if you click on the down arrow, at the end of the lens description, you can choose another lens to compare.

Please bear in mind, that some of these lenses are designed to be used on FF (135) format cameras, so you're limited to a camera body selection with lens.

Hope this is of some help to you, but I know it's hours of fun research to be had.

Also check this site out, may give you a better understanding of DOF againt Bokkeh, and how to achieve your desired results.

Oh and while I'm on a role, beside the great amount of information on the AP Library pages and How To's, you can always try here.

P.S. William W makes a very valid point re using your 18-55mm on the preset lengths you want. Trial and error here, may save you some of your hard earned.

Have Fun.

[Mr Felix]

Thanks folks.

Lots of reading done.

Am trying to do light painting as well, but it is getting the time, equipment and things all to align allowing me to go out and experiment.

Ok, what I have "learned".....

I shall probably use the incorrect names but it isn't done on purpose and I hope you can read around my honest mistakes and ones which may lead me to problems.

Looking at one site - won't post link as I am still not sure how Ric will take it - I was shown a good example of how DOF is used for "subject separation" and they went into it a bit.

It makes sense and I understand it.

As my lenses are 4.5 fastest, even 3.5 is beyond me. (that is talking all digital.)

So although my 2.8 "analogue" lens (or FILM) would only be 3.5 on the digital, it would be better than the 4.5 which I now have.

I also know that moving away from the subject shrinks the DOF, but sometimes that isn't possible, and with a fixed focal length lens, it will be harder again for obvious reasons.

So when I am out and I am wanting to take pictures of things and really draw them out from the for/back ground, it is difficult.

Sure, "post editing" can do it and I have tried. Gee did I cop an ear full on my efforts with them.

I'd prefer to get it right the first time as though using film, where post production isn't really possible.

So, yes, I understand that buying a "50mm" lens would be like buying an 85mm for the film camera, and all the changes to things between the technologies. But I can't change that. It is a constant.

I shall have to look if I want a "50mm" or a "35mm" and the F stop size.

I am going through the links supplied and found a few myself. So there is a lot of data to take in.


So, if you were wondering, I am still here. Hanging in. Waiting for the next meeting I can attend and enjoy - AND LEARN something.

That's always nice. And the people are nice too. It is good to catch up with people and "talk".

Anyway, I'll get back to reading now.

Bye.

[MarkChap]

It makes sense and I understand it.

But do you really ??

So although my 2.8 "analogue" lens (or FILM) would only be 3.5 on the digital, it would be better than the 4.5 which I now have.

NO, the aperture of a lens does not change, it is a physical property of the lens and nothing to do with the recording medium used behind the lens

I also know that moving away from the subject shrinks the DOF, but sometimes that isn't possible, and with a fixed focal length lens, it will be harder again for obvious reasons.

No, moving away from your subject increases DoF, as you get closer to your subject you shrink your DoF

So, yes, I understand that buying a "50mm" lens would be like buying an 85mm for the film camera, and all the changes to things between the technologies. But I can't change that. It is a constant.

Buying a 50mm lens is like buying a 50mm lens end of story. It is not like buying an 85mm lens for a film camera. The lens dynamics don't change ONLY the field of view changes
On your canon you will have the same field of view as an 85mm lens (or there abouts) but a 50mm lens will ALWAYS be a 50mm lens

[ricktas]

Looking at one site - won't post link as I am still not sure how Ric will take it - I was shown a good example of how DOF is used for "subject separation" and they went into it a bit.

You can post links to any site as long as it is relevant to the topic. Only members with less than 30 days membership and 50 posts cannot link to other sites. Neither of these two restrictions apply to you as you have exceeded both.

[Mr Felix]

But do you really ??



NO, the aperture of a lens does not change, it is a physical property of the lens and nothing to do with the recording medium used behind the lens



No, moving away from your subject increases DoF, as you get closer to your subject you shrink your DoF



Buying a 50mm lens is like buying a 50mm lens end of story. It is not like buying an 85mm lens for a film camera. The lens dynamics don't change ONLY the field of view changes
On your canon you will have the same field of view as an 85mm lens (or there abouts) but a 50mm lens will ALWAYS be a 50mm lens

Ok, maybe not 100%.

Part 2 was a bit of paraphrasing.

I read someone said that a film lens with F2 would be only a 3.5 on a digital camera.
Can't find it now.

Not familiar with how to look at older posts WHILE editing/posting a reply.

DOF, ok, got it the wrong way around.

So the CROP thingy/factor is the word - I think.
But as it changes what is finally made, I shall call it that for the sake of simplicity.

Yes it is wrong, but I am not doing it to be difficult. I am just not getting the EXACT words out every time.
I'm human. I make mistakes.

- - - Updated - - -

This is what I was reading then:

http://desmond-downs.blogspot.com.au...eparation.html

[arthurking83]

.....

I read someone said that a film lens with F2 would be only a 3.5 on a digital camera.
Can't find it now.

......

Totally incorrect!

As Mark has already said ... the aperture doesn't change, and using that same f/2 lens on either a full frame body or cropped body still gives you the same aperture and the exact same DOF!!!

This is why some of us try to be more specific about lens focal length and that a 50mm lens is the same irrespective of the format that it's used on.

A 50mm f/2 lens will give you exactly the same aperture value and DOF no matter what camera it's used with.


What you have confused with respect to that statement is that for the same field of view and subject distance, the DOF on the smaller sensor will be deeper for the same aperture value compared to the larger sensor camera body.

Also, for the same FOV and DOF, the smaller senor will give the illusion that the aperture value will approximate a smaller aperture on the larger sensor.
There is no exact ratio or percentage values associated, as DOF is not a fixed quality, but in general the rule is that for the same apparent DOF, the larger sensor camera will require an aperture adjustment of about 1.5 stops smaller.

What that means in actual usage is something like where a 50mm f/2 lens is used on a 35mm camera, for the same DOF and FOV on an APS-C camera you require (roughly speaking) 35mm and f/1.2.

Canon(1.6x) has a differently sized crop sensor to Nikon, Sony and Pentax(1.5x).


But for all intents and purposes, using a lens at f/2 on a film camera, will give you the same DOF on a crop camera too when set to f/2.
The only thing that changes in using the cropped camera is that you miss out on the periphery of the scene .. nothing else!

[ameerat42]

Mr Felix.
(I am reading this from far away, peering into a little "lapdog" screen and it's as slow as a wet week!) However...
If it helps, I will try to tell you "why" f-stops are constant for any lens, and whatever camera the lens is on.

(In think you know already that) the f-stop is just the ratio of the focal length over the aperture you are (actually, but don't particularly know) using. The reason for "not particlularly knowing" is that the primary aspect of the exposure you are trying to make is the "intensity of the light" reaching your film/sesnsor. There are two other factors (as you know) determining the exposure, and they are time exposed for (shutter speed) and sensitivity of the recording medium (ISO). (At this point, if you want to research T-factors, or transmission factors, that also affect exposure, go ahead, but for now the three above will do.)

When you swap any given lens with set f-stops and so set light intensity onto any other recording medium, that intensity does not change, even though the area it falls on may change. That's because the intensity is constant for any unit area. (Are you still there?)

That is the nub of it.

Now, if you want a spanner in the works, try to understand why the f-stop changes when you have a zoom lens and you change the focal length (by zooming in/out). In fact, you will conclude that the f-stop actually changes as you zoom in or out. OK, leave aside "constant f-stop" zoom lenses. They stay "constant" because the aperture changes.

HELP: draw a diagram, label it correctly, do some simple ratios, and Bingo! (Fr. "voila!", Ancient Greek. "Eureka", Colonial Australian "Stockade", etc.) There you have it.
A(I will now stop peering into this tiny compewter)m.

[Mr Felix]

Wow, what a can of worms.

So, here's the state of the nation now as for me:

I would like to do/take/make some shallow DOF pictures. The lenses I have won't allow that. 3.5 is just not fast enough.

Most of what I want to take is "medium range" pictures. So from fairly close to maybe 10 metres distance.

I still am not sure if I "want" a 50mm or 35mm, or what.

But looking at prices I am amazed at the difference between shops, even with the same lens!

Then there are the different brands as well. Haven't even looked into that part of the equation yet.

So really I am just as confused as I was at the start - which is why I haven't really put out any money to get one yet. (and partly because I don't really have money to "burn".)

I've seen prices from about $70 (au) to $150 for the SAME LENS.


Anyway, I'd better get back to researching the topic.

This is just to let you know I haven't given up.

[ktoopi]

I think you just said it Mr Felix! Research reserch and then research some more!! good luck