Manual focusing with a zoom lens

[NikolaSerbia]

Hi everybody,i have one question(its maybe silly). I have nikon d80 and 18-135mm,my question is,is there any difference when i focus manualy,and zoom in on the object that i wish to focus and move mu focus ring,oposite to focusing when my lens is closed on 18mm,because its a little bit difficult to see what is sharp. And as you all know,it does`nt have focal scale,so i have software on my mobile phone where i calculate DoF and hyperfocal distance.
Also one question,i love landscapes,can you recommend one lens which is very sharp and exppecialy good for that kind of shooting. I think that everything above 18mm would not be good,because i would get less DoF that this i already have. And on what aperture this lens works best,as i seen from my shooting,its f8 or 11,what about f20? two stops from max F.

Thanks to all who can help me understand this topic.

Best regards

[Dave Humphries]

Hi Nikola (and welcome),

As ever here, what might at first seem a silly question is rarely so.

For my part, and I may well be too paranoid here, I don't trust a zoom lens to be a zoom lens.
This is for two reasons;
a) my background is in television production; where it is common practice to zoom in, focus, then pull out to wider angle for the production framing (or shot). TV camera people spend time at each location checking the 'zoom tracking' to ensure this actually works.
b) My old Fuji S6500, although advertised as a 10:1 'zoom' lens, is not, it is (I would say), a 'variable focal length' lens (with a range of 10:1). The difference being, that for still shots, one doesn't necessarily HAVE to have it stay focused whilst 'zooming', whereas for TV (or movie) production, it is obviously essential. With the Fuji, and for all I know, some DSLR lenses, one tends to focus after framing, or in movie mode, have it continuously on auto-focus whilst zooming. I have never got out of the habit since moving to a DSLR of zoom (to frame up), then focus, even if wide angle, then take the picture.

I will admit to being lazy and using auto-focus rather than estimating distances, checking hyper-focal distances for adequate DoF, etc.
Is there a reason why, with the D80, you go to these lengths?

Too new at DSLR to advise on the second question, sorry.

Regards,

[NikolaSerbia]

Thank you for your reply. Well,i was just wondering is there any difference when I zoom in(135mm) on object which is about 50m away,and set my focus manualy,sharpen it and then close it to 18mm. Opposite to when i try to set my focus on 18mm on the same object ,when everything is much further and harder to see if there`s good focus.

[kevinbythebeach]

Thank you for your reply. Well,i was just wondering is there any difference when I zoom in(135mm) on object which is about 50m away,and set my focus manualy,sharpen it and then close it to 18mm. Opposite to when i try to set my focus on 18mm on the same object ,when everything is much further and harder to see if there`s good focus.

in my opinon the cameras auto focus will be more accurate and sharper than the human eye assuming all things equal.

[Colin Southern]

Thank you for your reply. Well,i was just wondering is there any difference when I zoom in(135mm) on object which is about 50m away,and set my focus manualy,sharpen it and then close it to 18mm. Opposite to when i try to set my focus on 18mm on the same object ,when everything is much further and harder to see if there`s good focus.

It's not a good habit to get into as most lenses aren't what's called "parfocal" (meaning the focus changes with the change in zoom) (usually not much, but I wouldn't do it).

A better technique if you want to focus manually is to use Liveview (if your camera has it) and zoom in 10x and focus from there. AF is usually more than adequate, unless your in very low light situations.

[William W]

Hi everybody,i have one question(its maybe silly). I have nikon d80 and 18-135mm,my question is,is there any difference when i focus manualy,and zoom in on the object that i wish to focus and move mu focus ring,oposite to focusing when my lens is closed on 18mm,because its a little bit difficult to see what is sharp. And as you all know,it does`nt have focal scale,so i have software on my mobile phone where i calculate DoF and hyperfocal distance.

It would be very UN-likely that lens will hold sharp focus at any one point whilst you zoom across the range. This has been mentioned already.

Also one question,i love landscapes,can you recommend one lens which is very sharp and exppecialy good for that kind of shooting. I think that everything above 18mm would not be good,because i would get less DoF that this i already have.

For any landscape work, I don't think focal length will impact on your hyperfocal distance until you are in the area well beyond FL = 30mm. Using a 30mm lens on your camera you can get about 7ft to infinity at F11.

As a general consideration, any Prime lens will be better for landscape work than the zoom lens you have. These are some reasons to consider:

> Zoom lenses are usually display affectations and aberrations at the extremes of their FL – you are using you zoom lens at the wide end for landscape work.
> A prime lens will likely be faster (larger maximum aperture, ergo a brighter viewfinder
> A prime lens will likely have a distance scale and maybe even a DoF scale on it.

Now the problem is there are very few VERY WIDE Prime lenses – i.e. very wide enough for wider than 18mm you already have for your APS-C Camera.

So, if you want really wide you might look at fast very wide zoom lenses like the various 10 to 24ish F/2.8 lenses available for APS-C cameras.

And on what aperture this lens works best,as i seen from my shooting, its f8 or 11,what about f20? two stops from max F.

I guess your lens is the: Nikon AF-S DX Zoom-Nikkor 18-135mm f/3.5-5.6G IF-ED.

Just following on from the point about zoom lenses showing their affectations worst at the extremes of the zoom compass, just consider the whole zoom concept – we are trying to design a lens to stay in focus keep an aperture and move elements to zoom in and out – very complex lens system so compromises are made.

In most cases we can think of a zoom being at it “best” in the middle of the zoom range and about three or four stops down from the largest aperture.

Now about three or four stops down from the lens’ largest aperture is often referred to the “sweet spot” – but really it is a range of apertures not just one – the same with the Focal Length “sweet spot” of a zoom - it is really a range.

So at an educated guess, you will get the best IQ from your zoom lens at about 28mm to 100 mm and around F11 to F16.

Now you have found your zoom to be best be best at F/8 to F/11 – but I would guess that is when you are using it at about FL = 18mm, F/8 to F/11 is about three or four stops down from F/3.5.

But I said your lens will likely show the best IQ between F11 to F16 ? ? ? that’s because I think at 28mm Focal length your maximum aperture will be about F/4.5 or F/5 – and at about 48mm the maximum will be about F/5.6

This is because yours is a VARYING MAXIMUM APERTURE ZOOM LENS.

If you stop any lens down beyond whatever its sweet spot range is – you begin to get into the area of diffraction – so NO, you will not be better to use F/20 with that lens.

In fact many Landscape Photographers will use a neutral density filter specifically to avoid using very small apertures – even on very high quality and expensive lenses.

WW

[NikolaSerbia]

Thank you all for advices !