Capturing the intricate details of astronomical objects requires mastering critical lens focusing skills. Without proper focus, even the most advanced astrophotography setup will fail to produce perfectly sharp images. While autofocus capabilities have improved dramatically, manual techniques remain essential for optimizing focus.
Rather than settling for “close enough,” astrophotographers must devote time to magnified focus adjustments, finding the exact focal point that maximizes sharpness. With persistence and the right methods, deep sky imagers can hone their lens focusing abilities to achieve breathtaking results.
Proper Lens Focusing Techniques
Achieving flawless focus for astrophotography requires learning and utilizing specific techniques and tools. Here is a more in-depth look at methods for ensuring tack-sharp stars in your images.
Live View Mode
Modern DSLR and mirrorless cameras have live view modes that enable magnified previews directly on the rear LCD screen. Enable your camera’s live view, then zoom in on a bright star to fill the frame. Slowly turn the lens focus ring while watching the star on the screen until it forms the smallest, sharpest point of light.
To carefully refine focus, view a bright star through your telescope eyepiece at high magnifications of at least 5x, preferably 10x or greater. Higher magnifications up to 50x can reveal even minor focusing errors. Gradually adjust focus until the star forms a tight pinpoint.
Precise Manual Focus
Autofocus systems can hunt back and forth unsuccessfully, unable to find the exact focus point. Switch your lens or telescope to manual focus mode to take full control. Turn the focus ring very slowly and deliberately, looking for the point of peak sharpness. Be extremely methodical and precise.
For wide field astrophotos using wide angle lenses, maximizing sharpness across the entire frame requires focus stacking. Take a sequence of exposures focused at varying distances and then blend them together in processing using focus stacking software. This overcomes the narrow depth of field limitation.
Infinity Focus Starting Point
When photographing stars, initially setting your lens just short of the infinity (∞) focus mark can provide a good starting point. But treat infinity as a reference, not the final goal. Make minor refinements around infinity to determine the absolute sharpest focus.
Finding the “Sweet Spot”
Identify and memorize the ideal focus point on your lens distance markings so you can quickly return to this “sweet spot” when needed. Marking the spot with a small dot of paint pen can be helpful.
Take the necessary time when focusing to utilize these techniques. Resist the urge to hastily focus your optics. Precise adjustments under high magnifications, not autofocus guesses, are the key to achieving lenses performing at their peak.
Typical Lens Focusing Mistakes to Avoid
While using the proper technique is critical for optimizing focus, astrophotographers must also be mindful to avoid these common pitfalls that will ruin image sharpness:
Attempting to focus while peering through the standard viewfinder or camera LCD screen often lacks the high magnification needed for truly critical focus required in astrophotography. Use live view digital zoom or high power eyepieces instead.
Trusting Autofocus Systems
Letting your camera or lens autofocus often fails to achieve optimal sharpness, as algorithms get confused hunting point light sources on dark backgrounds. Take full manual control of adjustments.
Accepting “Close Enough” Focus
Given astrophotography’s extremely narrow depth of field, being even slightly off the plane of critical focus is enough to noticeably degrade image sharpness and limit resolving fine details.
Not Adjusting Diopter
Failure to properly adjust a telescope’s diopter when observers change can introduce eyepiece-induced focusing errors. Always reset the diopter for your own vision.
Settling for Slightly Off
Being satisfied with stars that look almost sharp or nearly in focus will result in accepting mediocre results. Strive relentlessly for truly flawless, perfect focus.
Rushing the Process
Hastily working to get some focus rather than taking the necessary time to optimize it compromises outcomes. Meticulous, magnified adjustments are required.
Avoiding these common mistakes that often plague beginners will result in your images showcasing dramatically better sharpness and resolution of intricate details. The extra diligence pays off!
Additional Lens Focusing Considerations
Moving beyond basic techniques, astrophotographers need to factor in these additional elements that can impact focus:
Using Bahtinov Masks
These clever masks attach to the front of your telescope and generate a diffraction star pattern with spikes that simplify achieving perfect focus. The spike intersections align precisely at the ideal focus.
Accounting for Temperature Changes
As the temperature drops throughout a long astrophotography session, subtle changes in lens and mirror positions occur, shifting the focus point slightly. Monitor temperatures and be prepared to make small refocusing adjustments.
Astigmatism Aberration Effects
Optical issues like astigmatism can cause stars to appear elongated and stretched, even with proper focusing. Identify and address such aberrations through high power inspection, collimation, or optics upgrades.
Compensating for Field Curvature
Stars at the edge of the frame may require a slightly different focus point than those in the center due to field curvature in the optics. Take steps like focus stacking multiple exposures to maximize sharpness across the entire frame.
Allowing for Gradual Focus Shift
Even when perfectly focused at the start, the telescope optics are subject to gravity and mirror shifts that gradually alter focus over many hours. Periodically check focus on a reference star and make subtle corrections.
By understanding the potential impact of optical imperfections, environmental changes over time, and equipment-specific factors, astrophotographers can tweak their processes to maintain flawless focus throughout imaging sessions.
Lens Focusing Tips for Different Situations
While the fundamentals of optimizing focus remain constant, astrophotographers may need to tailor techniques and processes for different targets and equipment:
Wide Field Astrophotography
Composing wide shots of the Milky Way using fast wide angle lenses requires checking focus across the entire frame. Use live view to magnify stars in each area and focus stack multiple exposures if needed.
Deep Sky Imaging
Focus precisely on a bright star, then switch to frame the dim nebula or galaxy without touching the focuser. Exposure times are too long to autofocus on targets.
Star Trail Photos
With exposures potentially lasting hours, even tiny errors can gradually blur trailing stars. Magnify Polaris in live view and meticulously refine your focus for pinpoints.
As the temperature drops during a long sequence, the focus will drift slightly. Periodically check a bright star and make micro-adjustments to maintain consistent sharpness.
Tracking Sky Movement
When tracking objects like planets as they move across the sky, make very small focus tweaks during the sequence to keep the target precisely focused.
Astrophotographers must remain flexible and determine optimal approaches for their target and equipment. But the primary objective remains to ensure the most critical focus possible at all magnifications. Take the time needed to carefully nail focus in any situation. Don’t settle for anything less than perfectly sharp stars. Proper focus is a prerequisite for breathtaking astrophotography.
Below are answers to some common questions astrophotographers have about optimizing focus:
How can I accurately check if my focus is precise?
Use high magnifications of at least 10x when inspecting focus on a bright star. With truly exact focus, the star will form a tight pinpoint of light with clean, defined edges. Any fuzziness or elongation indicates unacceptable focus.
What level of magnification is ideal for critical focusing?
Good focus requires magnifications higher than your equipment’s native viewfinder or LCD screen. Aim for a minimum of 5-10x magnification, but high powers of 20x or 50x are ideal for distinguishing tiny focusing errors that still degrade sharpness.
Should I use autofocus or manual focus for astrophotography?
For long exposures, manual focus mode generally gives you the most control and consistency. Autofocus systems can struggle and hunt without finding the optimal focus point. Override autofocus with manual adjustments.
How often should I recheck and refine focus during an imaging session?
Significant temperature drop over long periods will subtly shift focus. Check critical focus on a star and be prepared to make slight adjustments about every hour or whenever the temperature changes by 5°F or more.
Is setting infinity focus best for astrophotography?
Infinity is a good starting point, but should not be considered an end goal. Slight differences exist between lenses and scopes. Make nuanced refinements around infinity to determine the absolute sharpest focus.
What causes the focus to shift during astrophotography sessions?
Changes in temperature by several degrees can alter lens and mirror positions by small amounts. Gravity shifts over hours as objects rise and set can also play a role.
Proper lens focusing for astrophotography requires sizable magnifications, patience, persistence, and avoiding assumptions. But the effort pays off with incredible sharpness and detail in your images!