Please explain DX lens verses FX lens focal length confusion

Have your say on issues related to using a DSLR camera.

Moderator: Moderators

Forum rules
Please ensure that you have a meaningful location included in your profile. Please refer to the FAQ for details of what "meaningful" is.

Please explain DX lens verses FX lens focal length confusion

Postby oavs on Fri Apr 09, 2010 12:11 pm

Well believe or not I am a reborn photographer. Since I had sold my last Nikon F4 and the rest of it since more than 12 years ago, I have decided to go back into to. So my gear is all new for the first time and all digital. Although software like Photoshop (and most of Adobe software) is a second nature to me and my daily work, DSLR is all new to me. I just picked up my new lens 105 macro DX. Love it hate it?

Love it because it is all new to me, just found out that it is actually close to 200mm instead of 105mm on a DX body . So as my 70-200mmf2.8 105 - 300 respectively. So annoying what they say is not what you are getting. I know they are actually a FX lens not DX... So is then DX 200mm on lens DX body equal to 200mm lens FX body? Apparently not.

Then why use those focal lengths.. they are confusing. If the the lens is manufactured for DX body then if I want to buy a 105 lens then I should view the same field of view through my DX body camera. I am aware that sensors are different size and that image gets cropped and all that, my point being if that is so then design the lens focal length and rate it accordingly. PS:I know I am specific to Nikon here not sure if Canon or others do the same.
PPS: I had to restart this topic from an recent tread at http://www.dslrusers.com/viewtopic.php?f=10&t=23222&p=411557#p411557
Gold Coast

Nikon:: 300s Body, Nikon 50mm f1.4, Nikon 10-24mm f3.5, Nikon 105mm f 2.8, Nikon 70-200 f2.8, Kenko tubes, SC-29 TTL Sync, SB-900, MB-D10 Grip, Lowepro SlingShot 350 AW, Manfrotto #190XDB, Head #141RC, Arm # 131DD, Magic Arm Set #143, CB Mini-Rc, and regretfully Nikon 18-55mm F3.5-5.6G
oavs
Newbie
 
Posts: 7
Joined: Thu Apr 08, 2010 1:02 am
Location: Gold Coast, QLD

Re: Please explain DX lens verses FX lens focal length confusion

Postby dawesy on Fri Apr 09, 2010 12:26 pm

The reason they use those focal lengths is because they are correct. Field of view (FOV), the amount a scene appearing in the image, and focal length are not the same thing. While changes in focal length will affect the FOV if nothing else changes, other things can affect it as well - primarily sensor size.

This is not a new phenomenon. If you ever worked with medium or large format (which I have not) you would have experienced the same thing. A 50mm lens on a 645 camera has a very different view to a 50mm lens on a 35mm camera. So to a 50mm lens on a 35mm camera has a different field of view to a DX camera.

Here is a quick article I found talking about this same issue with 35mm and 6x6 medum format
http://www.pentaconsix.com/35mmequiv.htm
[EDIT] This is a quick one of focal length, as you can see, sensot size is not part of the definition of focal length http://www.paragon-press.com/lens/lenchart.htm[/EDIT]

Essentially, focal length is a characteristic of a lens. Field of view is a characteristic of the lens combined with the image sensor size.

HTH.
dawesy
Senior Member
 
Posts: 681
Joined: Tue Jan 23, 2007 4:44 pm
Location: Roseville, Sydney

Re: Please explain DX lens verses FX lens focal length confusion

Postby Mr Darcy on Fri Apr 09, 2010 1:11 pm

Dawesy, your second link doesn't work for me. I get told I'm not allowed to go there.

As Dawsey said, focal length is a property of the lens, not the camera.
In simple terms, it is the distance from the centre of the lens (thickness not diameter) to the focused image, then the object being rendered is at infinity.
In real terms, take a lens, any (convex) lens. Move it back and forwards until the image of the sun starts burning a hole in the piece of paper under it. Now measure the distance from the centre of the lens to the paper (Quick now. The paper is burning!) This is the focal length.

You need to measure from the optical centre of the lens. For simple lenses (e.g. a magnifying glass), this is the same as the physical centre. Camera lenses are rarely simple though, so the optical centre may be offset one side or the other.

To get back to the real world, and your camera lenses. Get your trusty 50mm f1.4 off your Nikon F4. Put it on your D90.
It will work. I have done this (Well It was an 50mmf2, an FM2 and a D200, but those details don't matter) The field of view (What you can see in the view finder) will be different. Does that mean it has magically transformed into a 75mm lens? No. Nor should you paint the new number on the lens. The focal length hasn't changed. Now take your 105 lens and put it on your F4. It will work too, although you will lose a lot of the functionality, especially if it is the G series 105VR. The field of view will change, but the lens is the same.

My advice, forget the numbers. Get used to what you are seeing in the viewfinder. Photograph what you see.

What you are suggesting is akin to what Sony did when they brought out 200W ghetto blasters.
Were they more powerful than my home made 100W HiFi Amp? No. When you read the fine print, they were measuring in PMPO whatever that is. My amp was measured RMS into 8 Ohms. I do know what that is, though you may not. When I tracked down the conversion, it turned out that the ghetto blaster was about 5W when measured my way (the industry standard!)
Greg
It's easy to be good... when there is nothing else to do
User avatar
Mr Darcy
Senior Member
 
Posts: 3413
Joined: Thu Oct 26, 2006 11:35 pm
Location: The somewhat singed and blackened Blue Mountains

Re: Please explain DX lens verses FX lens focal length confusion

Postby dawesy on Fri Apr 09, 2010 1:24 pm

Mr Darcy wrote:Dawesy, your second link doesn't work for me. I get told I'm not allowed to go there.


Odd, found it via google and works in 3 browsers here... it essentially said what you said about the definition fo focal length, so oavs, if you can't get to it the info is in Mr Darcy's post.
dawesy
Senior Member
 
Posts: 681
Joined: Tue Jan 23, 2007 4:44 pm
Location: Roseville, Sydney

Re: Please explain DX lens verses FX lens focal length confusion

Postby gstark on Fri Apr 09, 2010 1:33 pm

dawesy wrote:
Mr Darcy wrote:Dawesy, your second link doesn't work for me. I get told I'm not allowed to go there.


Odd, found it via google and works in 3 browsers here...


I'm having what Greg's having. :)

Have you tried a different connection, rather than a different browser? This looks like it's tied to an ip address or some sort of login credentials issue.
g.
Gary Stark
Nikon, Canon, Bronica .... stuff
The people who want English to be the official language of the United States are uncomfortable with their leaders being fluent in it - US Pres. Bartlet
User avatar
gstark
Site Admin
 
Posts: 22659
Joined: Thu Aug 05, 2004 11:41 pm
Location: Bondi, NSW

Re: Please explain DX lens verses FX lens focal length confusion

Postby dawesy on Fri Apr 09, 2010 1:52 pm

hmmm... I can get to it via 2 Internet connections in Sydney with different providers and also from the US... Did a quick trace and it appears to be a hostgator site, that rings a bell as dodgy, so perhaps you gents have some filters protecting you from it? Perhaps I am just gifted today.

In any case, the pertinent info at the link is exactly what Mr Darcy wrote, so the OP has it in any case and hopefully it is helpful.
dawesy
Senior Member
 
Posts: 681
Joined: Tue Jan 23, 2007 4:44 pm
Location: Roseville, Sydney

Re: Please explain DX lens verses FX lens focal length confusion

Postby DaveB on Fri Apr 09, 2010 2:29 pm

The paragon-press.com site seems to be protecting that page based on the HTTP Referrer. Connections coming from this page are refused. If you go to the top of the site and drill down through Photography Tips to the lens chart it works.
And thereafter if you follow the link from this page it comes up OK in your browser, as your browser is using its cached copy and not asking the server for it.

Crappy website design, that's all. If it's going to refuse "deep linking" it should at least show a message explaining that instead of producing a cryptic server error!
User avatar
DaveB
Senior Member
 
Posts: 1850
Joined: Wed Aug 31, 2005 10:57 pm
Location: Box Hill, Vic

Re: Please explain DX lens verses FX lens focal length confusion

Postby tommyg on Fri Apr 09, 2010 5:02 pm

Focal length is the distance between the focal point of the lens and the digital sensor (or film etc) - this is the point of convergence of the beams through the lens.

A simple visual explanation of this can be found here;
http://www.dpreview.com/learn/?/key=focal+length

In the case of a DX body - the sensor is smaller, so it covers a smaller area of the image - so the image that is taken appears to be zoomed in more than on an FX body.

When they talk about a 70-200 mm lens on a DX body being an effective 105-300 lens, they mean that to get the exact same image on an DX and FX body you would need different focal lengths. Ie 100mm on the DX, and 150mm on the FX.

A 105mm lens always has a focal length of 105mm - no matter whether you have an FX, DX, MX or a hand-made camera at the end of the lens.

Simple really :D
Tom
Red Bubble
TLC Photography
Nikon D810, D700, D90 (IR)
tommyg
Member
 
Posts: 348
Joined: Mon Oct 27, 2008 11:36 am
Location: Mawson Lakes, Adelaide

Re: Please explain DX lens verses FX lens focal length confusion

Postby DaveB on Fri Apr 09, 2010 7:05 pm

tommyg wrote:When they talk about a 70-200 mm lens on a DX body being an effective 105-300 lens, they mean that to get the exact same image on an DX and FX body you would need different focal lengths. Ie 100mm on the DX, and 150mm on the FX.

In fact it's best not to refer to it that way.
A 70-200mm lens on a DX body has the same field of view (or angle of view if you prefer) as a 105-300mm lens does on an FX body. But it's still a 70-200mm lens (not "an effective 105-300mm"!).

In the same way, a 180mm lens on a 4x5" camera has the same field of view as a 32mm lens on a DX camera. But those are rarely used as standards for comparison: it would be more usual to say that each have the same field of view as a 48mm lens on a "35mm" ("FX") body.

A 105mm lens always has a focal length of 105mm - no matter whether you have an FX, DX, MX or a hand-made camera at the end of the lens.

Agreed.
User avatar
DaveB
Senior Member
 
Posts: 1850
Joined: Wed Aug 31, 2005 10:57 pm
Location: Box Hill, Vic

Re: Please explain DX lens verses FX lens focal length confusion

Postby oavs on Fri Apr 09, 2010 7:46 pm

Well thank you all for your kind response.

As I said I am new to digital and it will take a while to get my head around. This site recommended above explains well and I need to spend bit of time digesting it http://www.dpreview.com/learn/?/key=focal+length+multiplier

Thank you all...but this may not be the end of it yet, so take it easy on me :-)
Gold Coast

Nikon:: 300s Body, Nikon 50mm f1.4, Nikon 10-24mm f3.5, Nikon 105mm f 2.8, Nikon 70-200 f2.8, Kenko tubes, SC-29 TTL Sync, SB-900, MB-D10 Grip, Lowepro SlingShot 350 AW, Manfrotto #190XDB, Head #141RC, Arm # 131DD, Magic Arm Set #143, CB Mini-Rc, and regretfully Nikon 18-55mm F3.5-5.6G
oavs
Newbie
 
Posts: 7
Joined: Thu Apr 08, 2010 1:02 am
Location: Gold Coast, QLD


Return to General Discussion

Who is online

Users browsing this forum: No registered users and 0 guests

cron