Don't know what macro lens to get

[nerwin]

I plan on buying the Tamron 90mm f/2.8 VC. But I have a question about it and can't seem to find a solid answer. I know its normal for macro lenses to start stopping down from its largest aperture as you get closer..but at what point does the Tamron start stepping down from 2.8? I heard it was 20 feet! Thats just ridiculous. I thought you could focus at normal distances at 2.8? So if I want to take a picture of something at 2.8, I have to be 20 feet away from the subject? Just don't make any sense. I thought it was like 3 feet.

 

[jaomul]

Ya, i just checked it, bout 20 feet. Below 20 feet its f3 until about 3 feet when it starts climbing to about 4.5 at macro distance, most lenses do this, apparently not all lenses are seen by the camera to do this

 

[Derrel]

Well, loss of effective aperture at closer focusing distances is NORMAL on most macro lenses. Most macro lenses are going to lose about two full f/stops of effective aperture once the lens is focused to obtain 1:1 macro subject magnification...that is just the way it works.

The opposite alternative is to for the lens designers to design a lens that loses a lot of effective focal length, in order to eliminate, or to mitigate, loss of effective aperture. The problem with this second approach is that FRAMING issues become a royal female canine when the lens loses effective focal length in order to keep f/stop constant!!! Believe it: losing effective aperture value is the vastly preferred option in a macro lens design. It really, really,really is the preferred way to design a macro lens.

On the internet are thousands of posts, fixating on the most common fact that underlies the design of most macro lenses, which is the simple loss of effective aperture value as macro lenses are focused closer and closer and into the near-life-sized ranges. It's a lot like complaining that after you have driven a car 300 miles, the gas tank is not nearly so full as when you started the drive.

 

[nerwin]

I understand how macro lenses work..but I didn't think it started to step down 20 feet away from the subject! I've seen headshots taken with the Nikon 105 2.8G at f/2.8 and clearly it being a headshot, they are not standing 20 feet away. Must be just a Tamron thing.

 

[Derrel]

No, it's not "just a Tamron thing." MOST lenses LOSE effective focal length as they are focused closer. The difference is that with a macro lens, many people are using the lens in somewhat scientific, or technical, or industrial capacitites, and losing focal length as the lens is focused closer is not acceptable. It's a case of accurate and truthful reporting, as opposed to just hiding what's actually happening. My Nikon 60mm AF-D also drops to f/5.6 at MFD. Dozens of other macro designs do the same, exact thing.

The Nikon 70-200mm AF-S VR-G II loses a HUGE amount of focal length as it is focused closer and closer and closer. At its closest focusing distance, the 200mm setting is actually about 134mm in length, as I recall. Focal length is actually specified with all lenses focused at Infinity. The difference is that most lenses "hide" this loss of effective aperture by losing focal length as the lens is focused anywhere significantly closer than Infinity focus.

 

[nerwin]

Hmm, so why then when I focus at the minimal focus distance with my 50 1.8G and can still achieve the f/1.8 aperture?

 

[ruifo]

Derrel is right, all 1:1 macro lenses suffer of the same thing.
Non macro lenses with extension tubes loose their focus on infinity. As they gain macro level focus. Dedicated macro lenses don't loose focus on infinity, but at the expense of only focusing at the lowest f/# if at infinity or around. That's the trade off.
That's why longer focal lengths are also better for macro lenses, once bokeh tends to be better for portraits, even with higher f/#.

 

[Overread]

Hmm, so why then when I focus at the minimal focus distance with my 50 1.8G and can still achieve the f/1.8 aperture?

Because its not trying to be a macro lens and focus so much closer. If it was it probably would lose effective aperture/focal length. Even the Canon MPE65mm macro which is custom built do to macro and only macro loses effective aperture and focal length - at 5:1 its something like a 43mm f16 lens (I think - it somewhere around those values). Which is why the higher the magnification one uses on that the "wider" you set the aperture setting in the camera (unless you want diffraction to nuke all the sharpness)

 

[nerwin]

Thanks everyone for all the great replies. I think I understand how it works now..going to read up further on macro lenses. It wasn't a big deal, I was just curious why you can only get f/2.8 at infinity. But I understand now.

 

[Overread]

Yep just remember Canon uses will have f2.8 on their macro lenses - because we don't get the effective aperture reported to us. It kind of means if you're comparing canon to nikon shots in macro you have to factor in the f2.8 to f5.6 difference through the whole aperture range; this means Nikon users can use a smaller aperture and still get sharp shots; whilst Canon users appear to need a wider aperture to get the same level of sharpness.

 

[nerwin]

I did not know that, very interesting. So basically Nikon reports the effective aperture while Canon doesn't?

 

[ruifo]

To be honest, the macro lenses should read something like: 105mm f/2.8-5.6 (f/# varying with focus distance, instead of focal length), instead of only 105mm f/2.8 alone...

 

[Overread]

I did not know that, very interesting. So basically Nikon reports the effective aperture while Canon doesn't?

Yep, all of my macro lenses (including 3rd party which do report on Nikon cameras) only report f2.8 at the widest aperture even when at their min-focusing distance. The MPE 65mm macro even has a table in the manual for it which lists its effective apertures at its different magnifications.

 

[nerwin]

To be honest, the macro lenses should read something like: 105mm f/2.8-5.6 (f/# varying with focus distance, instead of focal length), instead of only 105mm f/2.8 alone...

I agree! It would make life much easier.

I did not know that, very interesting. So basically Nikon reports the effective aperture while Canon doesn't?

Yep, all of my macro lenses (including 3rd party which do report on Nikon cameras) only report f2.8 at the widest aperture even when at their min-focusing distance. The MPE 65mm macro even has a table in the manual for it which lists its effective apertures at its different magnifications.

Even though on Canon it says f/2.8, its not really f/2.8? Its more like f/3.5 or f/5.6, right?

 

[Overread]

At infinity focusing its f2.8
Its at the closer distances where it changes - just like the Nikon. You can know this for certain because 3rd party lenses (like those from Sigma or Tokina) which are identical optical formulas no matter their mount design; show the same behaviour. On a Canon they show f2.8 at the closest focusing distance; whilst on a Nikon they might show f5.6 (From what I recall some 3rd party might not report the change on a nikon)

It's purely a difference in how camera and lens talk to the photographer not a change in the actual optical properties.